Thank you ladyK, some great ideas there. Some of those I am already doing and certainly the point about observation I am sure is a very good one and your other thoughts I will try too where I can. Curiously my problem seems to be compounded by having too much room - if the cats were contained but closer together they would get more chance to feel safe in each other's company. He is neutered, but nonetheless clearly a 'top cat' and he was very close to my brother, so maybe cats can feel bereaved too.....
Anyway, I shall,persevere.