As per title, I got a cheque in the post....£150.00.
The payment form details simply says "Valneva Study Visit 14/05/21.
When I googled this it seems to say it is for taking part in "experimental Covid tests".
As mentioned, the Valneva study was a research project studying the efficiency of the Valneva vaccine. You can't accidentally take part in a clinical trial without knowing it.
What I suspect has happened is that someone with your name (and possibly even date of birth if you have a commonish name) has signed up for it and when the record has been looked up on the National Spine (PDS) they have linked the study to your record and not the other Dave7. Hence you got the cheque.
There should be a cover letter or note with the cheque. I would contact the department that sent it and let them know they have the wrong Dave7, particularly as this trial may get associated with your medical record and create confusion for you in the future (and for the other Dave7).