Thanks for your comments - so quickly as well - really appreciated.

Merlin has been eating mainly wet food, with one portion of dry to keep him going overnight (mainly as he bangs his food bowl all night otherwise!).

The vet has given me a supply of Hills Prescription pouches of wet food. Merlin is not very impressed but has eaten it. Having done a bit of research, I see this is also available on-line from Pet Planet (amongst others). I am told he may have to stay on this for life.

If anyone has any other advice about prevention/cure of this condition, I would be interested as Merlin is the only cat I have ever had - although I have had him since he was a kitten, I am still learning.