Could be worth checking the fuses on the LPG system, i believe some have fuses to protect the outputs which would include the vapouriser solenoid etc.
If one of them has blown it could be the solenoid coil faulty like Simon suggests although sometimes a fuse will pop through old age.
This was partially the problem.
With fantastic help from Edward at 'Mobile Automotives and LPG' the problem was diagnosed to be no voltage at the solenoid. The fuse was found to be OK but the fuse holder itself was burned in the area around one of the fuse legs probably caused by a poor electrical connection between the fuse leg and it's socket. Over time this poor connection had heated up until one side of the socket had burned out and disconnected the power to the solenoid.
A new in-line fuse holder and 15A fuse has been installed and this has cured the problem.
Once again thanks go to Edward for his excellent service.