Thanks very much for all the pointers. The dodgy flue seems like the
most likely candidate because we had a chimney sweep come around and
tell us that we needed to have it fixed. The lining had come undone and
it wasn't sucking up like it should do.

Its good to know though that its not normal for smoke and ash to come
up the door when refuelling, so Im gonna get the chimney sorted out
asap.

Thanks again.