The replaced seal from the last post did not solve the problem and a new gas exchange unit had to be fitted. This solved the problem of the disapearing coolant, but soon after I discovered a crack in the cylinder head (this was not present when we changed the head gasket, which we originally thought was the cause of the water loss, but it turned out to be the lpg system).
In short, the LPG system has:
1) Not been usable every winter, despite repeated attempts to resolve
2) Prematurely sent a battery to it's early grave (although I appreciate that car batteries are consumable items and this may have needed to be changed eventually anyway)
3) Caused a cylinder head to crack due to overheating as a result of the water loss. Whilst Z22SE engines have been known to crack cylinder heads, they do this around the spark plug aperture due to overtightening. This crack is on the flat metal of the rear of the cylinder head. This is an expensive problem even for an experienced mechanic such as me. For a non mechanic this may very well mean an uneconomical repair i.e. over £1000 parts and labour.