Gwyn; with his Army Number, you can apply for his WW2 Army records. This will give you solid information work on (see this, and many other threads for info on how to do that). With his PoW number, you may be able to get his PoW records from International Red Cross - but these may be very thin. In my experience (my father was in Stalag XXa and XXb), there is not a lot of documentation remaining regarding PoWs in Germany/Poland. Your GF may have completed a report on his captivity when he was repatriated (assuming he was). That should be in Kew. For me, the best source of information was my father's WW2 diary which describes life in XXb (and XXa) in great detail. I'm also lucky to have a few original photos of life and work in XXa and XXb. I visited XXb site in 2016 (Malbork, Poland) ... there's not much left except a memorial and a CWGC cemetery.
If you have any specific questions then please ask here.