However, I don't agree that you're entitled to more money for your unpaid overtime. From the mails, it looks like they notified you in advance that TOIL was off the table and therefore it was your responsibility to work hours that you deemed reasonable for your salary. I'm sure the company was very pleased that you worked so much OT, but it's clear that they did not require you to do so.
I also don't think you can justify keeping the extra months salary that they accidentally paid you. If your bank accidentally paid you a million bucks, you would not expect to be able to keep it, would you? What if you accidentally gave your buddy $100 instead of $10? You'd expect him to give it back, right? Sure, they screwed up; however they came clean about it and asked you to give the money back. They even gave you an extension to help you find the money. It sucks that you had already spent the money, but that was your mistake, not theirs. I think you're just going to have to bite the bullet and cough up the cash, sorry.
I'd chalk the whole think down to experience and move on. The games industry can be a very unforgiving place and now that you've seen some of the things that can happen, you can prepare yourself better for the future. There are many games jobs available in the UK at the moment, but, due to the bad economy, the publishers are all hurting for money. I would expect these kinds of situations to be quite common for the next year or two.