SORTED!
There's no such thing as a problem - it's just another reason to go for a run. Took the Mog for a run round the Mendips to warm it up.
1 when the engine's warm the gauge shows 95 degrees
2 back home both hoses are too hot to grasp
3 time to look behind the dash. Three screws on the front but a number of little black ones holding the fibreboard cover underneath the dash then...
4 While looking for these noticed that the louvres on the driver's side are in fact closed... (back to 3)
3 (cont'd) with the dash off it begins to make sense
- the heater control uses a bowden cable and has 2 quadrants - one with 6 holes & the other with a sprung ball for control
- turning the control shows lots of movement and 4 clicks (cable's too tight to reach the last detent)

So the problem was that the back of the control was fouling something out of sight behind the dash.

Put it all together last night taking care that the heater control has full movement. Then this afternoon just had to go for another run to prove it all worked in the real world. Hooray!