, good evening.
The cracking is more widespread near the chimney head, with some additional discoloration of the render which can be an indication of a long standing issue.
Was the chimney flue you now use for the wood burner at one time a "kitchen"? if so then the fire will have been on for considerable periods of time.
When the flue fell into disuse [say] if / when a central heating system was installed there will have been ample oppertunity for condensation to form on the inner faces of the flue, and the possibility of rain getting into the flue at the top.
What can / will / does occur is that the soot particles adhering to and ingrained into the flue walls are made damp by the formation of condensation, in turn the soot which is the result of burning fossil fuel which produces Sulfates will combine and produce a very weak Sulfuric acid, this acid attacks the old mortar and weakens the flue lining, and in severe cases can cause the stone on the external of the property to stain and even split and spall.
if there was a lot of such action on-going, the action of the [so called] Sulfate attack can at times dislodge and attack any applied render hence the render has / is seen to crack this crack line follows the line of the chimney where the effects of Sulfate attack has occurred.