Removed our bath and in place put a sealed shower unit, we cannot control the temperature of the water as it stays constantly hot, there is no pressure on cold water, so in order to have a shower we have to reduce the temperature on the combi boiler, surely not meant to work like this, do you have any suggestions for me

You failed to give a brand name or type of valve so I'm going to assume that you have a one handle shower valve.
Sounds like you have some trash blocking the inlet ports on the cartridge.
Shut the water off and open up the valve. Remove the cartridge and check and clean the inlet ports, Now, while the valve is still open, turn the water back on for a minute to flush out the supplies and open valve body. Reassemble and test. Better now? Good luck, Tom

Hi it's a mixer valve shower that's running of a combi boiler, its called an hawaiian shower unit, the temperature control inside the shower only does hot, and if you turn to cold it only trickles out, so am assuming there's not enough cold pressure, all the pipe work is brand new, the cold water pressure was fine before changing the bathroom, anymore suggestions would be appreciated

Thanks for the information. Did you do as I advised and what were the results? Back to you, Tom