In message , John Smith
writes
Thanks for your reply,

I've found out it's a Myson Midas wall mount combi

The boiler is 'working' as the rads are hot, the boiler seems to whirr up
into action as soon as the hot water switch is turned on, and it continues
to whirr until switched off again, but not change or click when the hot
water tap is turned on, and no hot water.
Off to investigate now anyway. Going to try and locate this divertor valve
and pressure switch first then...wherever they are.

** The Myson Midas has two separate pumped systems, one for hot water,
one for central heating. I don't think there is a diverter valve (it's
been a while since I tested any midas boards)

** The Midas also doesn't have an air pressure switch, it has an air
flow sensor which is basically two NTCs, one in the air flow, one not.

I would check that the pump impeller hasn't broken off the shaft first.
Then check out the hot water flow sensor.

Do you have a manual for the boiler, you'll need one. There are a series
of LEDs on the main pcb which indicates what the boiler is trying to do.

There are two versions of this boiler, one has a 404C147 main pcb
(usually blue), the other has a 404C656. They work slightly differently
to each other

Be warned, his is a complicated boiler, you might have let yourself in
for a little more than you first thought.

--
geoff