Mark
We did a similar job a while back in Newton Abbot, after some tw*t set fire to a parking meter !!
The smoke damage had gone up about 15 ' on the brickwork. We went to our local supplier, who make up their own cleaning solutions, bought a chemical for cleaning off carbon and sooty deposits.
We pre-soaked the brickwork, then treated with the chemical, then jet washed off, worked a treat. It came off so well, we did wonder if we needed the chemical at all! Worth doing a test panel first.