Not had that PM from you yet Dave, just found this thread myself ;-)
I've just checked your LPG system map, I didn't set a fuel trim in it but would have set one if for any reason I thought necessary.
On your LPG system it's possible to set individual trims for individual cylinders, if any cylinder was acting strangely in the way it responded to running on gas I would have first checked that no aspect of the LPG install was causing that (and changed LPG parts if necessary) or drawn conclusions about your car and written you a bit of a report on it. I didn't set any individual cylinder trims.
Not saying any of the following insights will lead to diagnosis of the problem but any of them might..
A failing lambda probe can stop giving proper signal on gas before it stops giving proper signal on petrol. Another thing that can bugger lambda probes is contact with engine coolant.
Your LPG injectors are on a rail but they are individual units on a rail... so (remembering your misfire codes for 4 cyls on the same bank) it's extremely unlikely 4 on the same rail would go wrong. But if one went wrong so causing dodgy fuelling (especially if dodgy fuelling led to a misfire on that cyl) it would mess up fuelling for the other 3 cyls on the same bank and could lead to misfires on any cyls on that bank.
Leaky petrol injector.
Duff LPG ECU could cause more than one cylinder to misfire straight away but very unlikely with your ECU... the output (LPG injector) driving electronics of some other LPG ECUs use common electronics for more than one output channel (so outputs usually cock up in pairs) but it seems your ECU (and most others) use discrete electronics for output channels so channels go down individually and I've never known a channel go down on your type of ECU.
Since you've had the manifold off... Crimped pipe between LPG injector and manifold, Crimped / shorted petrol injector break wiring, Routing (check plugs to LPG injectors and pipes between LPG and manifold match up... you don't have to remove anything except plugs to LPG injectors for this. Another indicator of problem routing is if it sputters particularly badly at some point like it's down to running on 6 or less instead of 7/8?).
Individual spark plug / lead
Compression or valve lift/timing on an individual cylinder