S4 heater not working

Ideas?

S4 - cooling system seems OK, thermostat functioning, fans operating,
can't
see a problem in that area.

Heater barely warmer than ambient and I suspect that what little heat
there
is comes from the warmth under the bonnet rather than from the heater
matrix.

I can attach a hose to the heater feed pipe and run clean water out of the
heater exit pipe - which indicates that there's no blockage in the heat
exchanger and that the heater control valve is opening. I can also run
water
into the heater inlet and thence through the system so that it eventually
appears in the expansion tank - which suggests that there aren't any
blockages on the route through.

So, as there's nothing wrong with it, why doesn't it work?

Any suggestions gratefully received.

-- 
PeterMcC
If you feel that any of the above is incorrect,
inappropriate or offensive in any way,
please ignore it and accept my apologies.