Bristan prism shower gone cold?

Bristan prism ceiling fed shower.
My shower as gone cold, when I move the temperature control it makes no difference, still cold. The pipes feeding it are fine, if I shut off the cold water the shower is hot. I have partially closed off the cold pipe at source so that the shower is warm as a temporary measure however the temperature still can not be adjusted with the shower control.
I did get a a new cartridge and put it in but it was the same, cold.
Any ideas?

Paul
November 2015