What's the most you could owe them? Assuming fifteen quid per month:
£15 x 12 x 2?
£360 tops?
I'd argue the point, though. If you are to blame for them not being paid, then it's a fair cop. If Tiscali are to blame, then fu
ck them, surely?
Were you paying them by Direct Debit? If so, why did it stop? Did you cancel it? Was it your bank's fault? Was it Tiscali's fault? Why didn't they send you letters after a couple of missed payments?
These are questions I'd want answered before I'd even consider admitting I owed them a penny!
Irrespctive of the outcome, I'd be leaving them and going elsewhere because if they can't send me a letter when there's no money being paid, I don't trust them to be helpful in other ways when you need it!
(And in resposne to the "should you have noticed?" questions, I only look for stuff that shouldn't be there or for new DDs recently set up, so I wouldn't have noticed either.)