Does anyone know what to add to gloss enamel paint to make it semi gloss or even flat? Thanks

you use a product calles a flatting base. Feast and Watson make one for enamel and there is also one for automotive paint

Thanks mate, soon as I saw your reply I remembered seeing that product at the shop I was in.

I believe that you can use diatomaceous earth. You would need a very pure and fine one and I suppose one that had not been calcined.