I have a Cav aged 14 months called Charlie. He is currently very fit, and has 2 35-minute walks a day. I have read about (and my vet has mentioned) the possibility of heart probelems in this breed, which is obviously a worry :( Is there anything I can do as an owner to prevent heart trouble? anything dietary? should he have lots of exercise, or not much at all?

If he has no heart problem now then keeping his weight right & not letting him get overweight is the way to go Mitral Valve Disorder is the heart problem & is more common in overweight Cavaliers

There is lots of good advice here from Cavaliers people

Heart problems in Cavaliers are hereditary. I had 2 litter brothers, one of whom developed a heart murmer. He was on medication during his latter years, but lived until he was 11. His brother, unaffected, lived to 14, but I think that's fairly unusual in Cavaliers. Obviously the prevention lies in breeders having their breeding animals screened for heart problems, and only using clear stock as far as possible. Other than that, avoiding obesity by moderation in diet and appropriate exercise, as in people, gives all animals the best chance of a long and healthy life. I have to say that the Cavaliers were wonderful dogs in that respect, thorougly enjoying their walks well into old age. :) :) :)
Cathy.

My first cavaliers father lived until 15 + & had a clear heart when he passed(cancer) You should not breed from dogs with MVD but even if the parents are clear doesn't mean their puppies will remain clear unfortuately. All dogs of course should be kept as fit as possible & not allowed to get overweight :D