cycling hot/cold is related to a flow issue.
next spep would be to check the shower hose is ok
and the inner hose is not collapsing and restricting flow.
check inlet filter.
check main stopcock is fully open.
is the pressure switch activating the pcb fully when shower cycling.
it's located at the bottom of the pcb.
these pcb's also suffer a bad solder connection on the microswitch on the pcb.
hi-spec plumbing & heating (essex)
August 2010