Johnny, yes the system should start on petrol for about two minutes till it's warmed up enough and then switch over but I'm thinking that an electrical fault is messing with the LPG ECU which is in turn not feeding the original ECU what it needs to even start up.
Mike, I haven't had chance to check voltages at the fuel pump yet but using the OBD yesterday I know thet the fuel pump relay was inactive the whole time regardless of ignition. Again as the OBD only shows a fault for the TB P1516 I'm thinking that may be because the fault doesn't really lie with the cars original electronics. Plus as I said there are no lights on the LPG control unit when I would have expected them. It seems the LPG system does tie into the TB , fuel pump and injectors so it should be possible for this to happen and to get the P1516 DTC.
While I've been looking into this I've been reading about LPG causing valve seat recession due to higher burn temps and lack of petrol lubrication - especially with aluminium headed engines! This wasn't something I read about or was told when I had my conversion done and now people are adding lubrication systems to mitigate this. Of course I'm thinking this could be the source of my long standing rattle. So even if I can find and sort the non starting problem myself (with the car stranded at my sisters house) I'm wondering about a new head, or seeing how much Vaux would be wanting to swap in a whole new engine or just selling it on for £5.00 and a packet of crisps and buying a shed. cryb..
Thanks for your input guys.
Kev