just changed the fuel/gas switch over in the el as it was broken and it displays the gas guage perfectly and changes fuel as it should when you flick the switch over. when i did this i put it on petrol for the first time in about a month and it ran like s**t so i used all the remaining fuel then filled it up and ran a tank through it and it was great again. when i went to change back onto gas, it just wouldnt start. will crank and crank and crank but no go. so to get it started on gas i now have to keep my foot down on the throttle till it finally fires. before i changed the switch it would crank 2 or 3 times then fire every time. now it still runs fine on gas and petrol and starts fine on petrol, it just doesnt start well at all on gas and i want to rectify this could it be the new switch that is causing the problem on start up or the s**t fuel that got ran through it for 100km has clogged something up or something... any help would be awesome

Firstly it is a very bad idea to start a car on LPG with your foot on the pedal. Will blow the a** out of your airbox.
now either you're gas mixtures are out. Have you fiddled with any knobs on the gas system?
Or your primer isn't priming when you turn the key on.
Or you need new plugs and leads.
When you turn the ignition to on leave it for 5 seconds and listen if you can hear a click with you're head under the bonnet. Can't always hear the primers tho.
Might pay to take it to a mechanic who fits/tunes gas systems.

yeh i know its bad for it but its the only way to get it going havnt touch anything to do with the gas system. i just unplugged the old gas switch in the dash and put the new one in. plugs and leads are both pretty new. car still runs fine on both fuels. its just getting it started on gas. im wondering whether its the switch thats not sending a signal or something that the old one did......... but really i have no idea haha

I thought most systems these days started on petrol automatically and switched back to gas by themselves! was the switch exactly the same or a different one?

Depends on the system im pretty sure.
Can you hear any priming?
Maybe try turning the key on wait 5 secs turn off. Do it 3 or so times see if it starts. (NO pedal action)