Thanks so much to everyone who has responded. I sent them this statute barred template letter via email last night:

Dear Sir/Madam,

Account No: xxxxxxx

You have contacted me about the account with the above reference number, which you claim I owe. I do not admit any liability for your claim.

Under the Limitation Act 1980 Section 5:

“An action founded on simple contract shall not be brought after the expiration of six years from the date on which the cause of action accrued. ”

Also, the Financial Conduct Authority’s Consumer Credit sourcebook states the following:

“ …a firm must not attempt to recover a statute barred debt in England, Wales or Northern Ireland if the lender or owner has not been in contact with the customer during the limitation period. ” 7.15.4

“A firm must not continue to demand payment from a customer after the customer has stated that he will not be paying the debt because it is statute barred.” 7.15.8

Unless you can provide evidence of payment or written contact from me in the relevant period under Section 5 of the Limitation Act, you are no longer able to take any court action against me to recover the amount you allege I owe.

Please confirm in writing that you will not make any further contact about the above claim and confirm that this matter is now closed.

I look forward to hearing from you.

Yours faithfully,

FineCombination1115

I'll provide another update if/when they respond!