Electric shower problems - flow/pressure?

We can't have a warm shower. Only icy cold. The low flow light comes on as soon as you use it and it doesn't heat the water. I can wash my hair over the bath with heat as there is enough pressure at the bottom of the bath but the minute you put it back into the riser rail holder it cuts out and only sprays cold. Do we call a plummer in? anything checklist type jobs we can do before we call one?
Moved into this house 3 wks ago, supposedly fully refurbed etc. ha ha ha so I can't say if the shower is brand new but it looks new (although the fact that I can't find the exact model makes me think it might not be new) It is a mira sport (looks like the sport max which has 3 pressure buttons opposed to a dial but isn't identical) My father-in-law (a sparky) said it's the only thing properly wired in the whole house.
House has a shared water pipe with another farm cottage, the actual farm & barn and apparently it also supplies a further big house. The flow is so poor from kitchen or bathroom taps (bungalow) my washing machine flashes 'check inlet' does all this point to something obvious?

Check your outside stop cock is fully open then the internal stop cock is as well.

Basics first.

If you are feeling adventurous, isolate the electric supply to it and check the inlet filter, you may have had some works done on the piping before you moved in & its clogged the filter up.

also you could check filter on the shower. as gas4you has sujested, in most cases poor flow not pressure can be the one.when you turn a cold tap on ,then another, does one of them go down to a trickle.

Thanks for the suggestions. Stopcocks checked open fully. The cold taps - yes they decrease, and one to a trickle when all three are on. Today I couldn't even get any heat in the shower when the hose was resting at the bottom of the bath.
Will go and try to check the inlet....
cheers neebs