Hi all, thanks for the responses, I was back at my family's over xmas and I forgot about this thread.
The GCSE at Manchester College does sound good, but its IF they run it again. I was going to start it in September just gone but they told me all language courses at the college apart from things like French and German had been cancelled, so I spent the money I'd saved on it. Then out of the blue and at very short notice, they told me it was in fact running, but I didn't have the cash to hand and I wasn't really prepared. So I don't want to count on that running again which involves waiting until September and then to possibly be told at the last second it's not running. Too unreliable, and ages off.
The SOAS postgrad course actually does look really interesting, so thanks for linking that! I'm definitely not going to go back to undergrad level now, aside from it just not being financially impossible I don't think I'd gain much from it overall.
I've got a friend who has been living in Japan for about 3 years teaching English, and while he loves it, teaching in a school is just not for me, I'm just not the kind of person who could do it. Also I hate kids!
My top choice at the moment is to go over to Japan and study the language there. What I'm looking at doing is an immersion course through a private school. Someone I did my night classes with is doing that at the moment and they gave me details of the company that sets everything up, so I'm currently emailing them for information. It would be expensive, but way less than going to back to uni would be, and I'd get much more out of it. I really want my main daily activity to be learning, rather than fitting it around work so I'm thinking of doing at least a 6 month course, possibly a year if I can afford it. Only thing is that it doesn't lead to a qualification at the end, so I'd probably have to do a JLPT when I got back to get something like that.
Basically, I'm not too concerned about becoming more employable, I just want to do what genuinely interests me while I'm still able to. If that leads to being more employable along the way then that's a great bonus, and I would like to work in some kind of translation/interpreting profession; I met someone from Japan who was doing it for Sony and it sounded like something I'd actually like to do, and this is the first step to that really. I'm not the kind of person who learns well in "spare time", I really need a classroom and teacher to learn properly, but that's increasingly hard for beginner level things as you get older. So, better to try and possibly fail at something you care about than waste my life doing jobs I've no interest in, right?
So thanks everyone for the suggestions, any other ideas or comments please post them. I'll try to remember to let you know what I decide on in the end.