I have a Triton T300si that is struggling to get hot enough?

Last Feb my T300si, 10kW developed a fault (I've forgotten what) so I grudgingly forked out about £140 for an engineer visit. He replaced the circuit board in the key pad. I noticed at the time that when he tested the shower he had the heat setting in the '20 to the hour' position which seemed a bit high but I thought nothing of it.
Now we are having to run it at maximum setting since the incoming water is colder. I am worried that when the weather gets really cold (it hasn't happened here yet) the shower won't cope even though it's 10kW. (Our previous one was 8.5 kW and, though the flow rate was not brilliant was acceptable.)
Since, as far as I can tell, temperature depends on flow rate are there any adjustments I can make. I'm going to phone Triton since it is not a year since last February but I bet the 'small print' will defeat me - 2 call-outs = 1 new shower!
Anyone have any suggestions?