My initial problem was no heat from the vents whatsoever (although with the a/c pump running I get chilled air out plus, when making various selections on the control panel, all sorts of flaps and valves can be heard moving behind the dash, and the airflow changes accordingly) so, after trawling through all the threads I could find on the topic, I changed the HBV and flushed the heater matrix (some brownish fluid came out but it was not particularly cruddy).

What I have now is a definite improvement in that the air output is warmer - but it is still far from hot. Plus, the heat output is erratic - with no apparent pattern between when it will deliver warm air and when it won't!

Two questions spring to mind.

1) Even though flushing the heater matrix appeared to show full and clear flow, could it still be furred up a bit? Is there any likely gain to be had in running some sort of commercial domestic de-scaler through it?

2) This is about a component I don't think I've ever come across a reference to here. It sits under the coolant header tank and appears to be some sort of inline pump (it is in the heater water circtuit - between the HBV and the matrix). If it is a pump, and it is not pumping (or not pumping consistently), could it cause the symptoms I describe?

I pulled the vac pipe and nothing changed - so it must be the water circuit, yes?

So, what's next? Do I have a go at a chemical de-fur of the heater matrix? If so, has anyone ever done this before? Any pointers as to what to do/avoid?

Yeah - descaling! Like what you do to a kettle or a steam iron.

I fitted a new HBV the other week and flushed the matrix out at the same time. Initially the water ran very slightly brown but very soon turned clear. I flushed it two or three times in both directions and the flow seemed ok with no apparent back pressure. I took all this as evidence that the matrix is ok - but it obviously ain't as I only get tepid air in the cabin (even with the HBV vac line disconnected).

What is this 'burping' you mention? I thought the whole system was self bleeding?

I'll give it a go - cheers!