Well, I gave up on insisting they were lying and that I did actually pay by direct debit on this account - they stopped replying to my emails.I'm not happy.
The electricity supplier (British Gas) changed accounts some time back after a new system came into force apparently, and they sent me a cheque recently for about £90 after that old account closed. Well, in the UK so many local banks have closed that it is difficult to find a bank as to where such can be cashed (the cheque is not from my bank either) - and given that I had a direct debit with them one might expect them to simply refund it via that account. Oh no, they are telling me that I didn't have a direct debit on this previous account, even though I have managed to find a statement online going back to 2016 showing that indeed I did pay by direct debit and have done for at least a decade.
We will have to see how this unfolds.
So, I found out that I could actually scan the cheque via my iPhone banking app - only to discover that it wasn't working today (first time as to such), seemed not to be able to take a photo of it, but that the cheque should be valid for six months - so I'll try later.