I'd go with rsnissan on this one.

Also if the heater element is really scaled up it'll trip the unit to turn the heater off (if they are trying to run it hot) and it'll come out freezing until it calms down again.

Take the head off and the cover for the unit (with electric off!!) and run the shower fully cold, give the heater part a bang with your hand and see if it's full of scale (will literally start flowing out the hose)

You can remove the heater part and clean the scale out (all be self explanatory when you see it with the cover off) but you will need to disconnect the water into it (isolate stop cock obviously) and disconnect the wiring to get the guts of the shower out.