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.

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