How can I get my Triton Combi shower to produce enough hot water?

My 'Triton Combi' shower no longer gets hot enough to take a shower. It is fed by a combi boiler which produces enough hot water for the sink/bath etc. I suspect the thermostatically controlled mixing unit is faulty but I'm guessing. The water pressures of both hot and cold feeding the shower unit seem equal to each other. I don't know the model number of the unit - it's about 10 yrs old. Any suggestions as to how to pin down the fault?