Im not getting hot water to the bath, how can i fix this?

I recently had a new shower fitted, and ever since then, the bath water doesnt run hot enough...only luke-warm or sometimes even cold! All the other taps in the house are fine. Has anyone else had this issue and if so how was it fixed? thing is...its an electric shower so shouldnt interfere with the bath should it? plus the shower is in a different bathroom. the only thing the plumber touched was the stop tap i think