Thanks to the folks who have replied. I’m in similar position, essentially won against bank. Debt firm has written, and I responded with one letter template from Marks workshop manual. Simple letter asking them to verify their claim, within 14 days, otherwise matter closed. Also has a fee schedule if they continue.
So next steps…Now I suspect they won’t take notice of that, so will Need to send them a default notice, then final notice, then cease and desist? Then start invoicing them?  So that I have evidence when I take them to court?  How long do debt collectors harrass?  Do they sell it on to other debt collectors? Appreciate any further advice.