Thanks everyone. Chimney is swept and clear. The one thing that is harder to clean is the top of the cowl itself. The brush is pushed down when it gets to the top so it wont clean the grill at the top of the cowl.
We do have an extractor in the kitchen and double glazing. But the extractor on/off doesnt make a difference and the double glazing hasnt changed but the fire behaviour has.
The only other thing I can think of is a leak in the flue, either in the flue, the seal between the flue and stove or if you have a register plate, a leak there, so when the hot air is creating a draw, it pulls air through the leak instead of through the stove. People always worry about leaks releasing smoke, but normally any leak will leak air into the flue system and kill the draw through the stove.
Well. Its had a good sweep and is totally sealed everywhere I can see behind the actual vents.
I cant get a picture to show you but looking closely at the cowling, it does look like an anti down draft one. But the aperture forvthe smoke is not very tall and looks like a circular piece of metal with slots cut out of it (rather than a simple mesh). I think this was a bit coked up. So i trimmed a chimney brush down to 6" so that at the top, rotating it would clean the inside of this area. When the weather is better i will go up and inspect it from the outside.
But for now, there is no smoke when i open the doors up with the flue damper wide open.