I took over an agreement from my deceased grandmother and updated the bank details paying for it, the name and the address. A new agreement document was sent out to me.

Now, someone else has cancelled that agreement and claimed a refund.

Are the insurers in breech od Data protection by allowing a third party access to my agreement or is therre some odd rule or twist which means it stays with the originla (deceased) applicant?

Don't think it would be a breach of data protection unless they'd disclosed details.

Im assuming this is life insurance we're talking about, but the rules are the same – the company should not have accepted instructions from anoyone other than the named policyholder – and if someone is impersonating you then thats blatant fraud