Hi dsms
as you may know the 5th was part of the 133rd Brigade, which in turn was part of the 44th Division. I haven't had time to do much digging but as a start you may like to read the following. It details the operations of the BEF, and mentions the areas that the 44th were located. Though most of it is interesting, you will probably find chapters IX onwards most useful.
HyperWar: The War in France and Flanders, 1939-1940 (UK Military Series)
As you say, if he was already back in Britain on the 28th it does sound like he had been wounded earlier, and evacuated. How he received his wound and where would have helped, as it was probably quiet a few days before he got back.