XJ40 heater misbehaving

Hi,
I bought a 1989 Jaguar Soverign XJ40 about 3 months ago and the heater has
been misbehaving since I got it.
The fan works ok at all settings but only produces any heat when I have it
on the highest fan setting (def) regardless of any of the heat seattings.
I wonder if there is an additional (electrical pehaps) element in the
demister setting?
Does the heater ECU die very often?
Any help would be most appreciated.

Well the issue with the vent system are a historical one.

1.)You will find flaps sticking and the cure is to get in there and
eliminate the binding.
2.)Second the servos that operate the flaps fail.
3.) The drains clog and the motor resistor card corrodes from the
pooling water.
4.) And to answer your question yes the ecu does die, although it could
only be the rotary switch assembly.

Also there is a vac valve under the battery; a failing diaphram can
give erratic heating.

DieInterim