Thanks for your replies!
- plenty of ventilation in the cabin, but a good thought
- there's the standard baffle in the stove, but no vent-reducer (can't remember what it's called)
- the flue is insulated so if it gets hot inside, most of the heat stays inside
- it's all new so no chance of build-up
Based on your replies I suspect that most of my problem is that I have rubbish wood at the moment (two years of floods, possibly a bit mouldy, it never really burned that well), and that's not something I can fix immediately! As I said, smokeless burns well (even wet), but that's a different story to wood.
I'll give a cowl a go, they look interesting. Also considering sticking an extra bit on the flue to see if that helps.
Cheers