+1 on ignoring LPG until you sort petrol running. One thing to check for either single or multipoint is where the LPG injector pipe(s) are installed to the inlet manifold. Make sure these are not allowing air into the engine when LPG is switched off. Google for "finding intake air leaks". Same process for any engine.
High tickover is most probably either Idle Control Valve or dirty intake tract especially inside the throttle body. Best to take it off & thoroughly clean all the crud out. (I had similar high idle few years ago, so cleaned MAF, Intake pipe, Throttle Body, and Plenum chamber. Been smooth ever since.)
As said above, check the O2 sensors are switching from 0V to 5V while running. Generic OBD reader should show the values. The 4-pin O2 sensor connectors either side of the block tend to get oily so clean these as well.
Thanks for this - have given a dosing of easy start all over outside of motor in appropriate places (no rev changes - should be no air leaks). Multiport LPG & all pipework is good/sealed.
Pulling the intake & ICV / MAF for cleaning this afternoon - have plenty of solvent, so hopefully some results here. Think I need to get a better OBD reaader.
As I said above in reply to kermit_rr:
"
Anyone able to advise ref diagnostics around South Herefordshire / Monmouthshire?
We can handle pretty much anything mechanical in the family without any issues - it's the knowing which components are stuffed which is now the issue & I've just been told to expect around £100 to plug in to a local LR garage's diagnostics at £80/hr plus vat with a minimum 1 hour charge.
Considering I usually play with old Chevy & Mopar V8's - normally needing a bigger hammer is the tech limits; I haven't had to seek diagnostic computery stuff in the last 15+ years, so I'm slightly out of touch with prices.
TBH, I'm getting tempted to throw a Carbed 350 Chevy or 360 Chrysler in here if I can't figure it out in a straightforward manner.
I don't want to simply be chasing problem after developing problem - especially when my son's best mate now wants the truck off me as he fell in love with it the first time he drove it!
My standards mean that it either has to be 'right' when it's sold, OR it has to be at an agreed discount to take into account any work needed as I WON'T rip someone off, even if I'm the loser financially (although I would like to minimise my costs/losses if possible)."
Sorry all, this one is becoming a Norse Saga...