My mixer tap could be faulty.It is a Mira Shower?

The shower is fed by a hot water tank. The water in the boiler is hot but the water coming out the shower is not very hot,even when I turn the mixer tap to the maximum heat setting.The shower was probably installed in 1980 when the house was built. The mixer tap is a Caradon and the part number is 1/100.There is also another number B.S.1415