Thanks everyone for the replies - hopefully I'll get my details, and my head straight - but I just feel so down about this..
I can't remember when I took out the card, but know it was quite a while back, shortly after Tesco brought out their finance arm of the business. I was a bit stupid when I was younger, and used money, particularly credit cards, as a way of trying to deal with my depression, and sometimes, even now, I feel like just hiding from things, but I am determined to get this sorted - I am just petrified that they'll demand this, and there is no way I can pay over £2k this month.
First I heard from Triton was this letter saying that I would have to pay the full balance within something like 20 days, which got me panicing.
From what I've read on this forum, my best step would be to proceed with the CCA thing, and take it from there?