She's been using a bit of coolant recently, not sure how much if I'm honest. I should have been measuring it. There's definitely a leak on the LPG reducer and the head -> expansion bottle return judging by the pink stains. However, today she overheated and took 1.5l of de-ionised water before the levels came back.

I've noticed what I think is quite a steamy exhaust - do you reckon this is a problem? The video was taken with a hot engine at 0 degrees ambient, running on LPG.

Video of a P38 steaming gently to herself on a garage forecourt

Did it puke out a load of coolant to lose that much? Out with the plugs to give them a look over can't hurt as a first check.

Hard to say on the steamy exhaust - if it were doing that after a run it would make me wonder. How long was it sat at the garage before restarting? The exhaust will cool off quickly when you stop the engine at a low ambient temperature, causing a load of condensation to build up when you restart and then boil off.

There was no puking, I think it's been a while since it was checked. We lost cabin heat then ended up stuck in traffic -> overheat. I pulled over immediately and waited while my wife brought the water :) (she did insist).
The drove to the garage (1/2 mile?) and took the vid while waiting for the LPG pump.

That amount of steam looks pretty normal in cold, damp conditions when running on LPG. Sounds more like you've got a number of small leak points and as soon as the level gets down low enough, you lose flow through the heater. Start looking for the rest of the leaks.

Both of mine produce steam like that on LPG at idle on a frosty morning. I wouldn't worry about it.
I use this stuff in my cooling systems. Together with a UV torch it's really handy for locating small leaks where the coolant gets blown/ cooked off.
Good for outdoor parties too
http://www.ringautomotive.com/uk/products/Workshop+Tools/Leak+Detection/Fluorescent+Dyes/RLD2

After my recent experience Morat, check your carpet and heater box lower parts/ trunking for coolant leakage. I'm assuming my failed Nissens matrix was a one-off QC problem, but you never know.