Its an increasing common problem with eco design stoves coupled with 5” flues and wide doors. Lots of instruction now advise to only refuel when the present charge has virtually burned away leaving nothing to smoke.
Hi, I know this is an old forum but I am having the same issue! did you get it sorted at all? Draw on the flue at the test point is great, nothing else seems to be wrong, but it smokes back into the room, we have the Farringdon too.