OK, after the 'we're sorry to hear you're unhappy' letter, I had two more (first one as a result of their 'investigation' into my complaint, and the second in response to my letter before action) drawing my attention to the terms of my account, blah blah. Standard letters.
I gave them until 10th April, which is this coming Monday, and although they've made it clear that they disagree with my claim, and consider their last letter to be their final say, I intend to give them until then before I file the claim. I am getting my stuff together to fill in the claim form online today, ready to file first thing Monday
Wish me luck... with interest etc, the claim will be in the region of £2700!