Medical records accessed and altered by "unknown staff member".

A report to the DWP should not be sent from your GP records unless you have given signed consent for it to be done. You also have the right to see the report before it is sent if you tick the right boxes on the form. Did you do this? Can't you ask the GP for a copy of the report that was sent and then get them to correct any information you think is wrong.
Do you know anyone personally who works at your GP surgery who would wish to change your record? Can you not discuss this with the Practice Manager? As for being sent for an unexpected procedure in another country this sounds so strange I would ask the surgery who referred you and why. As wildswan says it sounds as though they have got you mixed up with someone else.
Why dont you ask for a printout of your full computer data and check it for errors.

But you say you went to another country for a procedure you didn’t know you needed? Why would you go ? This a.l sounds so odd Didn’t you ask any questions like why do I need this procedure ?

I went for the appointment because it just said a scan was arranged and where. I thought it must be something routine because I was a lady of a certain age and I already had random mamagram appointments through the post.

Not another country, the next county :-D

The report was not sent from the GP, it was an anonymous call to the DWP in the same week, probably the same day, to say I was claiming fraudulently (I wasn't, nor even close).

I assume it was put there to bolster said complaint.

I have submitted a SAR to the surgery, I don't know how long that takes.

Sorry Damit my dyslexia strikes again and I misread county for country (or it might have been my dodgy eyesight ).