I have a '93 3.9 V8 Disco Auto with lpg. Problem is that it will start and run perfectly one day and absolutly refuse to start the next day.That precludes any fault with the hot wire unit and bypass air valve both of which have been cleaned,there is a good spark,the fuel pump runs I have even jumped the injector relays for some reason it appears that there is no signal to pulse the injectors.I have even gone as far as disconnecting the lpg emulators on one bank to prove that they are not interupting the signal.I have even gone as far as changing the alarm module and also removing the alarm relay(that got me home once courtesy RAC but the ploy has'nt worked since).Also checked the inertia switch seems ok.Daft thing is it will start after a bit of pursuasion on lpg and run ok problem is I've now run out of Lpg and vehicle is stuck at home so it has to run on petrol to get some more!!.Anyone any sensible ideas

I take it that this a single point injection due to the age of the vehicle?
You will have a fuel/LPG c/o switch and LPG LED level unit -yes? is this selection switch selected correctly and only set for LPG? Newer multipoint injection set ups only allow start up on Petrol - possibly some fault in your c/o selection relay not allowing petrol running?