Any question answered...

There is a decent updraft to draw the draw up the chimney with a window open (thanks for diagram @Clockwise). Perhaps the room is warm enough, I didn't have to resort to a beer can doobie to get it moving. Did notice the steam from my tea followed it up. @mdcc_tester how many steamy cups of tea compare to a log fire for spliff extraction purposes?

Attempt two went pretty well but attempt three introduced a whole new vector to contend with; nipping on the highest grade with a coffee this morning in starlet position on the floor, legs cocked, head supported and wrist flopped over coquettishly holding an elegent, ivory coloured shortie turning brown at the fat end, I was startled by a distant cooing before my draft vanished and instead of the pure sour tang of the goddess I got a cold waft of smelly old chimney air.

I think a pigeon might have sat on my chimney pot.

Maybe the convective encouragement of tea is greater than coffee? Maybe a gently grass-warmed chimney pot is irresistible to a pigeon's bot? @NurseHolliday do you suppose a larger ting ting will scare it off?

Bit worried tbh, a pigeon came down a girlfriend's chimney once and flapped around for ages and did a poo on her duvet so don't want to accidentally lure them in....