From the IMDb: "In this episode the viewer discovers why Morse has a limp in later life. A storyline from the young Endeavour Morse to coincide with the real-life injury sustained by John Thaw and thus a physical element that Thaw brought to the character of Inspector Morse."
This type of situation always reminds me of James Doohan, who played Scotty on 'Star Trek'. Doohan gave the middle finger to Hitler (as he put it) when he lost the digit during the Allied landing at Normandy. But they never addressed it on 'Star Trek' - in fact they covered it up in one episode where Scotty had to lay his hand on a truth verification device by using a "stunt hand". (As you can see from this screecap from "The Trouble With Tribbles", however, they weren't always successful.)
They should have addressed the issue when Scotty returned in "Relics", the 'Star Trek: The Next Generation' episode. For over 70 years, Scotty had been in the "storage" of a recycling transporter beam. When he was rescued, Scotty could have had a bit missing that couldn't be reconstituted and hence, the loss of his finger.....