teutonic

Just to conclude this story (like anyone cares)
 
Doing this (though varying between having it on / off recirc mode, having the vents open and closed) has done the trick.
 
'Honest John' reckons the cause in these instances is condensation inside various ducts etc, in which bacteria grow, causing the smell.
 
The heat (and rapid change to cold) kills the bacteria and the heat dries out the ducting.
 
So in this instance nothign to do with the air con (which makes sense as the smell disappeared when the air con was on)
 
Apparently the air con itself can give a similar smell when bacteria get in and around the condenser, or the drain / pipe etc.
 
I think in my case the problem was partly caused by the car sitting a lot in early / mid summer (was riding the bike to work a lot).
 
So I broke my own rule i.e. knowing that cars don't like sitting on the driveway and not being driven.

So answer = 'there's a reason why there's a market of providers offering to do this for you"
 
Thanks for the useful observations about outside temperature and being on the move - will try those out.