I understand Lowells have recently bought a lot of debts from Barclays, & that they admit many of them are statute barred.
This does not mean that the debt ceases to exist; only that they cannot take Court action to get payment (unless, as already said, there was a CCJ before the 6 years were up).
You can tell them you can't or won't pay & that may be the end of the matter; or they could be difficult & go on chasing you. If they do it might be worth agreeing to pay a very small sum in full & final settlement of the debt. But if you do this, their agreement must be in writing before you make the payment, & it is safest if the payment is made to them by a third party - not direct by you.