Hopefully i can help..
In regards XXXXX XXXXX loss..how much is being lost roughly or often do you need to top up?
Any warnings on the dash coming up ie: overheating etc.?
Any excess vapour/steam from the exhaust?
What tests have been done..and i asume by "your already tried" apart from the cap and oil the whole coolant system has been replaced/installed?
yes coolant system has been replace
slight overheating the first time
May be slight more vapour from exhaust.
Vauxhall are trying things under warranty
Ifeel it is cylinderhead gasget
That may well be a possibilty in regards XXXXX XXXXX headgasket..although they will need to do the proper tests first before confirming fully the head hasn't warped or cracked or head torque setting incorrect or headgasket is at fualt/breached - tests usually inc:- cylinder leak down test/cylinder compression test/cooling system leak test to apply pressure to the cooling system and monitor the guage for pressure loss.It could well be this fault will only show up once the engine has reach operating temperature and everything expands but doing the test above should be able to show that up or simulate that.If you have as you say the excess vapour from the exhaust and does smell a little like coolant/anitfreeze then its does sound very like head or the headgasket is at fualt here.seeing as all other areas such as the entire coolant system being replaced bar maybe leaking inside the car via a fualty matrix..However i would stick to your guns and explain that the fualt is still present and get them to check that fully in regards XXXXX XXXXX tests above and see what their findings are..