Landy Newbie with LPG issues

Hi everyone, just bought my new Disco (well new to me) and it's a petrol / LPG conversion. It's been ok for the last few days but today it's decided not to use the LPG when I switched over, but continued to use all my petrol instead. I'm new to LPG and I'm not sure if I've done some thing stupid or not?!? I start on petrol, wait for the engine to warm and then switch over. The little Amber light flashes to indicate warming up and then once I have my green lights showing four bars the Amber light goes solid and the red light indicating petrol use goes off, it did the same today but did not use the LPG, just my petrol!!! Any pointers or advice would be great...

best try and find out what system you have to start

thanks for that, trying to find out and will post once I have an idea!!

Its an OMVL single point LPG system. I have had it looked at but it seems the switch has gone wonky, does anyone know of anyone selling the OMVL switches??