The temperature control on the 240 is only for the central heating. The hot water is pre-set on the p.c.b.
There may be something wrong with the burner settings but this will need an engineer to check it.
One thing you can try is to speed up the flow of water through the hot taps so it has less time to pick up heat. If this does'nt work there is'nt much else you can try. Most of the other stuff needs a corgi registered engineer to test and adjust or replace.
T.h.s, July 2007