Ok although it ran fine on my test drive and was fine yesterday, it wouldn't run properly on LPG again today. Idles fine on LPG and runs fine on petrol but never backfires.
I'm going to assume that the LPG filter is clogged (or something else in the line) and therefore can't supply enough LPG.
I've closed the tank valve and I'm going to take it in for an LPG service now that I work nearby to where I had it installed. Won't be til January sometime so til then it is running on petrol for me :)

