Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?

As well as weird effects due to the combi design, another possibility if the
combi DHW output and the CH pipes run in very close proximity (particularly
if boxed in) is that the DHW is being heated by the CH pipes whilst sitting
adjacent to them, to a much higher temp that the actual DHW output. When you
turn on the tap, this slug of hot water comes out.

It is only a suggestion, and probably isn't the problem, but could be fixed
by separating the pipes with a bit of insulation.

Christian.