Thank you for your reply.
It is odd,
It was not for a new card, change of address or high priced item.
There is no explanation. So perhaps as you say some random security measure !
My bank could see the £1.00 taken with the code (to link card/bank....not sure which it was tbh) and they could see that was already being returned, but could not see that for the mysterious £1.00.
It is just a worry, with scams etc.
The bank just said keep an eye on tge account :))