The most likely sources would be exhaust gas or anti-freeze. Both have quite a distinctive smell though.

Exhaust gas could be getting back through a poor tailgate seal & anti-freeze could be escaping from the heater matrix.

Try putting a thin smear of petroleum jelly on the tailgate seal & see if it transfers to the opposite surface when closed. Try running the car without the heater on before it gets too cold.

Thanks for thoughts and ideas.....

Air con = off... so think can cross that out
No water leaks or top ups needed, under bonnet... so think can cross out antifreeze.

leaves the seals as a good area to look at.... there is (what I think) a wierd design with this car.... in that the exhaust tail pipe ends around a foot short of the back of the car.... but apparently it is correct exhaust and vauxhals design for 05 estate?... could be a contributing factor in itself.

Getting a wiff of the taxi - car- bus in front is fairly common place.... but this situation occurs in car, in it's own space.... more a slow build/effect

My desire..... is this..... to somehow find a gadget or selection of gadgets or test paper (personal badge type thing).... that will detect and reveal the "air quality".... what the #### I'm actually breathing

I'd love to go to Halfords an get something that hangs on the rear-view mirror.... and turns black if the air is fumey... sort of thing

I'll continue search.. cheers all