My 9 month old cat was sick last night three times. It seemed to be just food that came back up.

This morning he appears to have had very bad diarrhea whilst out and about and has come home rather smelly and wet so I have had to bath him.

He seems OK - his usual sleepy self!

Has anyone else experience this before with their cat?

It could have just been something other than the cat food he ate that disagreed with him... a dead mouse or something else yukky.
I would be taking him to a vet as soon as possible though in case he has eaten a mouse that has been poisoned or has something else serious wrong with him.

In the mean time make sure he drinks plenty of fluids as diarreah can be very dehydrating.