Purchased an assortment of grabbers, magnets on sticks etc but the fishing trip was unproductive. Took plugs out & turned engine over with car in gear by pushing it but still could not find anything so with trepidation I fitted the new sensor & started the car. Everything was fine with no sound of tortured bolt against fly wheel. Since I was now 4 hours late leaving for Norfolk I decided to risk it & the car ran perfectly the whole journey. One last fish through the clutch arm aperture the next day before starting the Hethel track day still failed to produce the missing bolt. Track day & 200 mile journey home completed with no incident so I can only assume that either the bolt didn't fall in the gearbox bell housing, it's lodged somewhere inaccessible inside, or it's got stuck externally on the engine.