Only tepid water on mixer shower?

We have just moved into our new house and the shower doesn't get hot. As far as I can tell, it's a simple wall-mounted mixer shower (don't know if thermostatic or not) fed from both hot and cold tanks, with a single dial that is supposed to make the water hotter the more you turn it round. On maximum the water temp is only just warm. But there is nice hot water coming out of the basin tap next to the shower. Unfortunately there is no make/model information on the shower itself so I can't look up details of what it is or how to adjust it.

Any suggestions before we call out a plumber? Thank you!