[AndrewA]

From memory the only thing on the back of it is the tail of the heater matrix.
I/we took the whole heater box out as we were redoing the seals, cleaning up the fan motor, refurbishing the matrix and so on, so it was easier to manipulate when out of car. I'd suggest being gently brutal twisting the control valve left and right but perhaps wait for a second opinion.
Good news that you've identified the problem though.
When working through the car last year I tended to get a lot of my parts from SLJ, Pegasus or Bristol Classic Car Parts (in no particular order).