Thanks for the info and suggestions. Have been back to the vets, and everything seems ok. He's lively enough (the dog not the vet!) and the coat is ok too. The suggestion was made to mix some puppy food from the same manufacturer in with his normal food, and this does seem to be working - slowly, so we'll carry on with this for awhile. Once a week I now just go and pop him on the vet's scales and they record it on his notes, so the situation is being monitored! It feels very strange trying to put weight ON a spaniel.........