Hi again, thanks for previous help, much appreciated. Can't be HG as system not pressurising too much, no mayo on dip (though there is a little at the top of the filler).
Is there a cure for what appears to be plain, fresh water gathering in the nearside head around the plugs? Would this cause a misfire (cutting out!)  on lpg when first transferring from petrol? After topping up coolant yesterday to too high ( left cap off to allow any air in system to bubble out) then going out for a run, got heating back but it took about 6 miles (mostly uphill) to get running with only slight misfire on lpg. After a short stop, (10 minutes at most) transferred onto gas very quickly and ran beautifully, though still a little rough at tickover. Bearing in mind my lack of facilities, skill, knowledge, tools etc. are there any recommendations where I can take the car to get all this fixed, preferably in Fife and NOT Wellside Motors? (I know they're Perth & Kinross but they're still nearest lpg specialist I know of). Thanks for any help, will be back at pc by 13:00.

Sounds like you have rainwater leaking from the wiper scuttle and running into the plug wells.
Are you losing coolant ? If so, the HBV could be leaking and the coolant is spraying into the plug wells ? :-/
Mayo on filler, short journeys (particularily in cold weather) will cause this.
Water in plug wells will definetely cause a misfire - especially when running on LPG. ;)

i just had a similar problem, mine was a pin hole in the coolant pipe that runs along the nearside plugs and coolant got into the wells. i changed the plugs and leads on this side and all missfiring went away. however its now leaking somewhere else. so yes albs is right, i took off my wiper scuttle and noticed a previous owner had used a clear silicon to seal where the two parts join. i can see how this would gather water at the plugs.not sure where to get fixed as ive been doing mine myself but struggling too. good luck

Thanks again guys, will check out these suggestions. Again, will post to say how I've got on. Cheers! :)