fred wrote:

Thanks for the details Andy, the one I look after has just developed the
same fault, H/W only when C/H is already on, I'll make sure I have the
diaphragm when I get round to looking at it.

You get those symptoms when the diaphragm starts to fail; the arm moves
enough to give partial operation of the diverter valve, but not enough
to activate the DHW microswitch. IME it deteriorates rapidly after that
and you'll soon have no HW at all, even if the burner is already firing
for CH.

--
Andy