I bit the bullet and ripped - carefully removed the dash out last weekend and installed my new genuine Blend motors and the heating is working perfectly.
No book symbol - first for me on a P38

DO NOT connect the blend motors before installing them !!

Dash board top front panels are not a taxing job too do, time consuming working out where everthing has to come out, some of the plastic ducting had been cut by previous owners and some of the 'joiners' were missing but I just duct taped them back together and there are no air leaks.
Advice to anyone doing this job - take your time, take photographs, be very careful removing the flimsy plastic panels, a good head torch is a must, good quality trim removers and screw drivers, good quality duct tape.

The 4 Bolts holding the dash top under the windscreen are easy to remove with the plastic ducting out, I did not fix the plastic ducting to the dash top when installing it, the bolts are a pain to reinstall, I ground the bolts to a point as this makes reinstallation soooo much less stressful and saves you time.
As my car is a 'rolling restoration work in progress' there are stiil loads of plast panels to replace so the job is still not done but I can drive the car safely.

I replaced heater matrix supply pipe O rings while I was 'in there', the O rings were brittle and fell to bits.

Hope this helps anyone.
Message if you need any help.
Onward
Ian

I did my passenger side the other week. Removed the airbag, drilled a hole to unscrew the duct, got angry with the air duct and finally got it out of the way. Cut the cable and re-solders the potentiometer. Reattached with a choc block and works fine now.

Planning on doing the drivers side soon but not looking forward to it as I think it's supposed to be harder. Hopefully it's just another blob of solder.