Thanks Andy, a useful linky.
I found the demist actuator right up high above the glove compartment and against the center console. I unbolted it and had it in my hand as I went through the range of heater direction settings and it seems to be intermittent at best, I couldn't get it to repeat any of the small movements it made as I cycled through the options.
The vent moved freely and I could divert the air upwards to demist but it took a 90 degree movement to do this and the most the actuator would move was about 10-15 degrees, so I suspect it's on it's way out.
Until I get hold of a replacement I have a 'Winter' and 'Summer' setting ... in Winter I tie-wrap the vent open and in Summer i cut the tie-wrap !
;o)