Here is my tale of trying to get it back.

£50 isn't a large amount of money to a massive organisation like Sky. But for me, it's two to three weeks food, or a train ticket to visit my mother, or about half of my Christmas shopping (yeah alright, I'm not very extravagant with that sort of thing).
Following is an account of how a relatively small mistake on their part, which I thought was resolved well and quickly, then led to an agonising four month back-and-forth of:
a) Lies

b) Broken promises

c) Incompetencies

d) All of the above.

May: Attempted cancellation

I messaged through Help & Support to cancel my broadband and phone line, because I was going away a lot over the summer and it wasn't worth keeping it on. I was told it was sorted. I went to Australia for a month.

June: Apparently failed cancellation

I got back to Edinburgh and discovered I'd been billed. I turned on my router to see if I'd managed to mess up the cancellation myself, and saw no sign of broadband. So the services were cancelled, just not the billing.

18 July: Good customer service

I finally got round to contacting Sky about my bill; by this point I'd been billed for July as well.
I spoke to a couple of people on the phone who made me feel like it was my fault, then finally to the lovely Rachael who dug deeper, discovered a human error had occurred as a result of (we think) the two ways of writing the first line my address: 2F2 35 or 35/5. As a result, I had two account numbers... one had the services I guess, and one had the bills. It was very confusing, but once we found the two numbers Rachael properly cancelled my mysterious second account, and very kindly applied £50 credit to make up for the overbilling. I was told to contact again in a few weeks to initiate the refund process.

16-26 Sept: Being blown off

I tried several times to contact customer services via help tickets to ask for the refund but got told (by a human, not an automated error message) there 'system error' with my bill, or something totally unhelpful (telling me something I didn't ask about) in response.

29 Sept: Actually, no

I'm told the £50 credit can only be spent on Sky services, not refunded directly to a bank account.

29 Sept: I damn well disagree!

I protested the unfairness of this, since I have moved somewhere with an existing broadband provider and actually probably wouldn't be going back to Sky after all this even if I had a choice...
I was told that actually they can refund it after all, and I should expect it in 48 hours. I pointed out that I'd heard this before. I was told not to worry, it'll be fine! I relaxed.
48 hours later... no refund.

Social media

Throughout September and October I started tweeting about the problems. I liked to do things like compare Sky's customer service to that of Virgin Mobile (who have done wonderful things for me from time to time). I hoped 'public shaming' might speed up a resolution. I got encouraging responses from the social media team at Sky, who actually seemed to care (which is their job, I suppose) but really I only ended up opening more tickets and talking to other advisors in the end.

21 Oct: A change of tune

Following another help ticket chase up, I'm told credit was incorrectly applied in the first place, and has been removed. I'm not exaggerating (just paraphrasing) when I say this was done "for reasons".

21 Oct: You what?!

Oh, hey guys, how about... no? I didn't just suffer through months of torment for you try to tell me the only competant member of staff I have spoken to was actually incompetant after all!
So I had a very long live chat to William (I think; I've named him, because he didn't leave a follow-up note like he was supposed to) who "carefully reviewed my account" and agreed with the verdict that the credit was incorrectly applied. I protested. He "carefully reviewed my account" some more, and then said he could see that an explicit reason was left for the application of the credit to my account in the first place... so it should have been there after all! Yay.
He said it would be refunded to my bank account in 78 hours. I told him I'd heard this before. He said not to worry, it'll definitely be fine this time.
Guess what.

31 October: It wasn't.

So far, no refund, no re-application of credit to my bill, and not even an update to my help ticket about the conversation. It was like it never happened. I stupidly didn't think to copy the chat as evidence, although I assume they have a transcript of it somewhere.

1 November: Hope

I DMed the social media team a bit, and scheduled a time to live chat with one of them, directly. While I waited, I typed out a timeline of everything that happened so far so I could paste it straight to them.
Three quarters of an hour later, I have hope once more following a chat with the most human member of Sky customer services I have spoken to so far.
He found actual reasons for things that had been done for "reasons". For example, my initial refund was never successfully issued because goodwill credit must be issued as a cheque, not a bank account transfer, so Finance just rejected it. A silly rule, but that's the way of it. (In my case the response was just to not issue it though, so Finance can't even follow their own silly rules).
I'm still not entirely sure why the credit was totally removed though, or why on my bill its removal shows up as a charge for Sky TV.
He treated me like a person by not making vague promises, or holding back particularities of how the organisation works. He told me he believed I should be receiving a refund based on what he knew so far, but he'd have to talk to his manager. He talked to his manager, who agreed. But he didn't then just tell me everything would be fine. He told me it still might get rejected by Finance (for "reasons", I presume).
What he is doing is speaking directly to someone in Finance to get a cheque sent out. He's manually changing my address to make sure the cheque goes to the right place, and he's going to get in touch with me again on Wednesday night to let me know what the progress has been.
I asked him what the next step is if Finance reject the refund request, and he implied threats of violence. (NOTE to Sky managers who might read this: I don't believe he meant he would really commit violence on the Finance team. He was doing his job well and using humour to relieve me whilst promising he would make an effor to follow up).
He also gave me permission to verbally abuse him if he doesn't get in touch on Wednesday night. I appreciate this sentiment, though it's unlikely I'll get into capslock territory with this guy any time soon. I would tweet a gentle a reminder of course.

6 November: The Wednesday follow-up

I got a Twitter DM with a link to a live chat... After just under an hour of waiting in a 'queue' for the live chat I had to leave, and DMed @SkyHelpTeam back to ask if they'd let me know when there was someone there, so I wasn't waisting my time refreshing a page.  They responded and sent the details to my MySky help tickets instead.  And the result?

A cheque is in the post!

Please allow 28 days for it to arrive.

I sure hope that's true.  And that it's coming to the right address.  I might send a letter to my old flat, just in case.  So I guess I'll update in 28 days whether or not it actually arrived.  I'm hopeful, but they've promised me that money is on its way in x amount of time before...

Conclusions

If your problems aren't fixed immediately, pester the social media team (@SkyHelpTeam).  Get everything in writing, record every conversation, keep track of dates, names of customer service people, and what you were promised.  Don't give up.  For every semi-competant and sympathetic customer service person, there are four or five lazy/useless/uncaring or possibly even malicious ones.  Just keep trying, and you'll get through eventually...

Recommendations for Sky

Following my unfortunately extensive experience with Sky help ticketing, I'd like to make a few suggestions for its improvement.

1. Tickets should be marked as resolved by the customer. I have so many tickets that I don't consider to be resolved, sitting in my 'resolved' tickets column.
2. I should be able to reply to tickets. I post a request, I get a response that is marked as resolved that I don't agree with. I then have to follow up by opening a new ticket, which inevitably goes to a different person, and I end up going around in circles.
3. If I'm taking the time to type out messages to you, it probably means I don't want to talk to you on the phone. It doesn't matter why. Take the time to write messages back. (Related: telling me it's free to call customer service on a Sky line is really unhelpful when I'm trying to contact you about a recently cancelled Sky line. The fact that I never physically had a landline phone, line or no, is irrelevant here).