Cannot control temperature of Mira Excel shower?

I have recently replaced my boiler with a new combination boiler and discarded the pump that was used in conjuction with a Mira Exel shower ( 1992). Since the system is now under pressure there was no need to have the pump. However I cannot control the tempreature of the water from the shower it appears that the tempreature from the shower is the same as the setting on the boiler and cannot be cooled. If I try cooling , the shower just shuts down. Any idea of what may be wrong.

Les
May 2009