Thanks everyone. It's been a bit of a shock as I only knew he was unwell Friday night/Saturday morning.
He was suffering from heart failure. His heart was very enlarged and his lungs full of fluid. The vet (Ruth) did say it is possible to treat with diuretics but she had never seen one that bad from the outset and felt he was so bad today that it was unfair on him to try as success would be unlikely and he was really struggling. I can't get my head around how he can have been so poorly but managed to hide it from me until the weather warmed up suddenly. Apparently cats and rabbits are quite good at hiding this condition.