Further to this, the problem appeared to begin when the engine bay was treated to a steam clean coutesy of a spraying with water from the hose that provides water cooling to the LPG reducer.
The distributor and coil unit were in the direct line of fire, but both have been thoroughly dried out and appear to be OK. I would have expected the fault to affect LPG running as well as petrol running if the fault was electrical.
I know my description is a bit vague, but I've been running over this in my head since it first occurred and have, as a consequence, got a bit fuzzy headed with it.
Any ideas guys?