Any idea how much a new diverter valve or cylinder stat. costs (approx?)?

cylinder stat £10
diverter valve depending on what on £45.00 - £85.00

check the diverter valve first, which is located above the heating pump.
poss near your hot water cylinder in cupboard.
on the side check for a lever and push it across a few times it may be sticking.

hi-spec plumbing
September 2007