Smoke escaping for back

[Lookingintoart]

We suddenly have smoke seeping out of the back and side of the range.
we've had an Esse Woodburning Cooking Range for over 6 years with little trouble. It is regularly cleaned and we got the sweep in the check it but he had no idea.
We don't seem to have changed anything - what else could it be?

 

[brad wilton]

I assume the sweep checked flue for blockage,maybe clogged how has your outside temps been have you had problem with any temp drop outside could be cold flue preheat oven with newspaper then light small fire see what happens