Vaillant Combi VCW 25/1 T3 Hot water temperature lowish?

My boiler has been in operation since 1983 and I have replaced various parts over the years so am fairly happy to tackle most jobs. Everything works but the hot water does not get as hot as it used to as the boiler modulates to low flame most of the time the hot water is on. Turning the flow down from the hot tap improves it a bit. I suspect the hot water heat exchanger may be getting coked up as I live in a hard water area.
Before I tackle the heat exchanger are there any other thoughts that may help resolve the issue?
Thanks in anticipation.

Roy
October 2008