Sorry, I don't think i made myself very clear earlier. It's not that he thinks it's hard to do it was more me just being inquizitive about the process.
The only thing that was flagging up the possibility of head gasket failure was the liquid test which definitely had turned a slight yellow/green colour that indicates hydrocarbons (I think that's what he said?). One Lambda has been changed, 2 others where showing intermittant faults but seemed ok on test and the 4th was fine. The bloody warning light has come back on again today after it was reset last week. It's weird because it seems to come on at the same time every trip, cruising at around 70mph on the motorway!
With regards to losing coolant, I'm certainly not noticing any obvious drop in levels, although there may be only a very small amount leaking through the system I suppose.
So just to clarify (sorry if I'm being a bit slow here!), If I get the compression test done again and the readings are all normal, then it definitely indicates that head gasket is ok?