When did it last have a new set of plugs? Often a fault will appear when you are running on gas and not appear on petrol so the assumption is that it has to be the gas system - this isn't always the case though!
Because LPG is harder to ignite than petrol, the plugs have to work harder so tend to have a shorter service life than normal. On a "normal" 4-pot engine like the Astra with easily accessed plugs, the chances are it will be cheaper to buy standard plugs and replace them more often with only the inconvenience of actually changing them to worry about.
On something like a FWD V6 with the rear bank close to the bulkhead and the plugs buried on the side of the head rather than the top, maybe fitting standard plugs more often isn't such a great idea!
Recently i had a Lambda sensor go and i could make it happen every time, without fail, running on gas giving it some beans. It had to be a gas problem bringing the MIL on, right? Wrong!
I switched to petrol and reset the ECU, gave it the beans and got a red herring - it didn't bring the MIL light up!
So i gave it a few more beans and lo & behold the MIL popped up like a little red monster!. Reset the ECU and gave it a similar amount of beans, MIL back on again.
Checked the ECU - 2 flashes - rear bank Lambda sensor.
It had a new pair of sensors (front and rear banks) about 2 months ago for the MoT as one of the original sensors (front) failed during the emissions test.
The point of all this waffle about Lambda sensors is don't discount something just because it is new/recently fitted and often the fault is there on petrol as well, just needs slightly different conditions (like me giving it a few more beans to get the Lambda to play up) to show itself.
If your Astra had a set of plugs recently, what make were they, is the gap correct and if you move the plug from the cylinder with the fault does the fault move to the new cylinder?
Most people go for NGK or Bosch plugs as they tend to be the most reliable - personally it's NGK as 1st, 2nd, 3rd and 4th choice for me with Bosch coming a close 5th.
Could well be worth throwing a new set of NGK plugs at it, especially if you're unsure when the plugs were last changed - it won't do any harm and will eliminate one of the most obvious causes.
Let us know how you get on!
Somewhere in Suffolk with a Jeep, 2 Rovers and a V6 Volvo