Thanks very much everyone, I think this might have been solved! Yes, it does sort of look like Inval. (I always thought it began with an I anyway) and it does make total sense that in the Discharged column it would say Inval. before the illness or injury. Suppose I could scroll through a few more naval records at some point to see whether that's standard procedure anyway.
So yes, it seems he died as a civilian, but pleasingly, he has still been included on the local War Memorial. He was only mid-30s so I can't help wondering whether he got sunburned a few too many times on deck - presumably as a stoker, he would want to take every opportunity he had to see daylight!
