The heater control has gone on my black series 2 VSX, so no heat and until recently no fan either. Having tried three second-hand units the best we've been able to achieve is getting the fan running constantly on high speed. It's tolerable now but will obviously be hopeless in Winter.

New price is apparently £385 + VAT, so I have to find a serviceable used unit or part with the car.

Anyone have any ideas please?

I'm thinking straight away that it might be the heater motor itself is playing up as the "high speed only" symptom is typical of that unit. It's behind the fabric-covered panel above the passenger's footwell. The connections get corroded and the transisters are noted for their unreliability after a time as well. I have S/H unit which may the answer to your problem. Let me know after you have exhausted all the other options and proved the motor to be U/S

....and the flap servo motor under the glove box, right hand side, also fail so you can't change the temperature.

Peter.N.