Has the problem just started since the plugs were changed? Do you know what plugs were used?
I have GM 4 electrode plugs in mine. They are over 30K old including 12K on LPG and no problems at all. No gap adjustment required on these plugs.
When you say popping to you mean backfiring through the intake? If so this points to a lean condition. Does the problem occur at idle or under load? What type of LPG system is it? Mixer or SGI?
Sorry for the barrage questions. Just trying to establish what we're dealing with. 
Kevin
Kev the car has a long history so here it goes...
Rob bought the car then had the Gas fitted. He says it run great with no problems at all He then got an oil leak on the drivers cam cover, so took it to a garage to have it done. The Cambelt was done also at this garage. From this point on it didn't run properly. So he took the car back to the Gas fitter where he found that the Garage had put the wrong connectors on the gas injectors.
Still the car was running wrong so then took back to the garage where they say the car is running correctly.
I checked the Cambelt Timing and it was hardly out.
The EGR Valve has been blocked off.
ICV cleaned out.
Dispack and Leads I have checked with known good ones, still the same.
I found that the gearbox breather was on the breather bridge, the breather pipe that was supposed to be on there was hanging down at the back

The car was ticking over better but almost asthough it was missing very slightly.
Rob wet home the rag me as when he dropped his mate off, he got a clunking when the car changed gear. I reckoned it sounded asthough the gearbox needed oil. I had a word with Mark he mentioned something about with the gearbox breather being on Vac it would have given a false reading or something like that, plus with the oil being low the torque convertor would be putting load on the engine which could effect the tick over....
Anyway Rob has topped up with gearbox oil and the clunking has gone and the car is running lovely on Petrol but like shit on the Gas.
As for the Plugs, they are Vauxhall plugs with the double electrodes on.
Pretty sure Rob told me that the gas is Sequent. Valve type = Europa. Reducer Type = Genius
Rob also says it is backfiring through the inlet when under load, but only when its on gas.