We left the back of the fireplace behind the stove like this for the chimney sweep to see - it has gone black. He said a seal had gone and resealed it with fire proof cement. Mark re-painted it this evening while I was at college so it is clean again.
£40 for the chimney sweep and £25 for the fire proof boarding and Fred is now good to go for another winter!
This evening was week 9 of college.