Having reset the MCB my electric shower is now not heating the water fully, why?

The most likely reason why the unit is not heating the water fully is that a heating element has failed. Which would cause the mcb to trip.

Note: If an element has failed other parts within the unit may have been affected it is therefore important to check all the electrical circuits in the unit for signs of failure, Depending on the shower type and cost of replacement parts we might recommend a replacement shower.