Hot and cold water do not seen to be balanced after repair

Hot water fine ..no heat to radiators. So replaced a myson Motörhead on my vented heating system..

Heating working fine, but the water does not seen to be balanced at the shower or bath.

I know that while I was waiting for the motorhead to arrive via an online plumbing supplier. A plumber looked at the system and managed to get the heating on ( although on for 15 mins...) he said to keep the hot water on and the thermostat in the lounge was set to max.

Any thoughts on this issue I would be grateful of.

Heating system have nothing to do with hot and cold water system.

How do you mean, not seem to be balanced at the shower or bath?

Do you mean there is more pressure on cold than hot?
Daniel.