See a previous posters reply below. This is probably your best bet for accurate info and a starting point for further searching
The MoD will only release service records to a next of kin. So you will need a) proof of grandfather's death (death certificate) and b) that you are his direct next of kin.
The MoD will send you a "Certificate of Kinship" form which you have to complete and send along with the documentary proof you are your grandfather's next of kin. If your grandmother is alive for instance, the law is that MoD would only release this information to her and not you. There is a fee of £25.00 also.
You can download the "Certificate of Kinship form from here:
Contact address for Polish service records (don't worry that it has RAF in the address; all Polish service records are archived here) at MoD:
APC Polish Enquiries,
West End Road,
Tel: 0208 833 8603
Fax: 0208 833 8866
e-mail: email@example.com or firstname.lastname@example.org