My Uncle was a Jap POW, in Java and then Changi jail in singapore.
In his case, he had kept all his war time papers, many of which were deposited at the Imperial War Museum (IWM), after his death.
When I contact the IWM a few years ago, then sent me photocopies of everything they had free of charge. This included his medical record, weight, etc, so I was able to find out quite a bit about his time there.
The point being that the IWM may also be a place to contact.
Cheers!