Thanks guys.
I should have said, I am a B737 Captain, but as you say, I'm very conscious that it IS their turf, and that small issues can be taken out of context or be a catalyst for bigger problems over time.
I mean, I'm happy to politely ask the FO not to light up for a few hours, but I don't want it to result in me having to view my coffee with some suspicion thereafter !
Cheers.