[AndrewA]

Excellent - thanks for that - yes I know the control!
But thanks to you I now know what I've not done as it were. Refurb'd heater box now back in car but as doing it single handed, took a guess at how much etc flaps were opening when tightening up cables.
As most driving since then done on cool evenings, have not really noticed heat until yesterday and assumed I'd got it spot on first time!
I'd forgotten I was going back to adjust cables at a later date when another pair of hands were available.
Made classic mistake of ascribing it to a "new" issue - instead of unfinished job.
Doh - as Homer would say
Schoolboy-ish error that should be a simple fix.. with another pair of hands.
Thanks for the idea/nudge in the right direction.
There was a Britannia on the car run as well and had a long chat with a gent who had owned a variety of Bristol cars over the years but had finally settled on a Bentley Turbo R as his run around classic. He was definitely missing his Bristolling days.
Andrew