Kim, don't know if you made progress but you will need to go to Kew. There are records for Plymouth and Dartmoor Prisons for French prisoners. There were over 100,000 over the years and the ledgers have details of rank, name, age, ship, or battalion, build and colouring. It was quite a revelation to see so much detail.
Have now taken up another subscribers suggestion and have found my target on the Vincennes website. Hoping the document gives me the information we want on Prosper Combes.