faulty shower mixer tap?????

[llamorna]

i have replaced my bath taps with a shower mixer tap. The problem is that the shower part works wonderful on cold water. When i attempt to run the hot water to mix warm water the shower diverter switches to pour the water in the bath.
Even if a mix the water in the bath outlet first then pull the diverter to use the shower it switches back to the bath outlet. Is this a fault? How can it be fixed?

Thanks Julie

 

[Softus]

I have a fiver here that says you have mains cold to the tap but that the hot is what's known as 'tank fed'.

 

[hi-spec plumbing heating]

recon so