Baz
This is a very common problem encountered by many people.Most shower or tub faucets come with a built-in anti-scalding device.In most faucets,this device could be a small rod or ball which moves from left to right regulating water pressure and the hot/cold mechanism.
This device probably has debris collected inside preventing it from balancing the pressure and hot/cold water demands.Cleaning it will help.If this does not help,try removing the handle of the device,you will find a plastic ring which you will have to re-install.
It is better if you can get the help of a professional plumbing service.
