Hi all. My first post so I hope someone can help. I have no heat whatso ever. I have replaced a non existent thermostat so I now have 90 degrees. I  have hot water to the lower feed to the matrix and coolant flows from the top hose when disconnected. I understand that there is a solenoid  control valve some where. It is not behind the firewall and cannot see anything that resembles a valve in the engine bay. because of all the different types not entirely sure of what it should look like either. Coolant refilled and bled with no apparent air locks.
Loosing hope and do not want to spend an unnecessary fortune on a car that has various other issues, but it is a good very clean car and would like to get it all right so as to use as my last car. (old bloke)
Looking forward to your replies.
Max      (and if anyone needs help with MGB's I'm your man)

Max.

If you are expecting a solenoid valve on the coolant system you will be disappointed. Cabin temperature adjustment by control of coolant flow was abandoned years ago. Maybe you already know that, but I mention it just in case.

Temperature regulation will be by mixing hot and cold air by means of flap valves in the air ducts. I don't know if the flaps on your car are entirely mechanical or electrically powered. Note that electric flaps are known to be prone to failure.

There is also the possibility of a partial blockage of the heater matrix which can be caused by scale or by crystals of silica gel that have escaped from a bag in the coolant reservoir. (Bags of silica gel in the reservoir were installed for a few years by Audi.) I am inclined to think the latter because such crystals also prevented the thermostat from closing, which may explain why yours was missing. There are videos on Youtube explaining the silica gel problem and how to change the heater matrix if necessary.

Thank you for your answer. I was hoping for a nice simple solenoid valve that could easily be replaced or bypassed. The matrix does sound more likely though as I get heat to the top pipe, (presumably the input)  but the lower remains cold. I will try a backwash. Will look further when I have addressed the dpf issue.  Thankyou again.

You have nothing to lose by back flushing the matrix, ideally with 1 bar or more pressure, though don't be too optimistic of success. Let the flushing water go to drain in case it does push out some solids.