RR 4.0L P38 Misfire

Hi all
I have a nice 2001 Range Rover 4.0L P38 with LPG fitted
It will start on petrol and then run sweet as a nut on LPG BUT, not now, run very well on Petrol at all.
Its been on various diagnostic systems and has had new injectors O2 sensors as indicated and two new coil packs, but still misfires.
I've been told it could be the engine wiring loom or the mass air flow sensor but I just don't know
The engine is absolutely mint when on LPG
Does any one know a place not to far from LE67 where I can get it fixed
Thank you!

Freemans Common, Leicester.
Land Rover specialists | Swinfield-Cooper Ltd

ETA Used them many times over the years for various parts and quite a few brain picks when I've been stumped!

Thanks for that!
I used to use them when I had a 110 a bit dear but they know their stuff!