Thanks , Geoff. Every little helps!
I realised that William had died between 1841 - 1861 and had found a couple of possibilities on BMD. East Stonehouse Dec 1844,and Plymouth Dec 1858. One of these is now eliminated.
I shall now follow up Julie's lead but, if William's age in 1841 (53) is anything like correct, it looks as if he joined up when he was 12! Interesting.