After a brilliant Swanage Folk Festival, I got my box home and, to my horror, heard a slight hissing sound. Nightmare. A few seconds' listening suggested it was coming from behind the grille. So off it came.
Somehow the wax on the pallets had melted and allowed a valve or two to slip out of place. The wax melts at a very low temperature (a bit too low in my opinion). I carefully used a soldering iron to warm the armature (not the wax itself!) in short bursts. It took very little heating indeed to allow me to move the valves back in place.
Job done!
If you do have this problem, use this solution at your own risk and remember just how little heat the wax needs!
