Coolant system pressurising but not overheating?!

[D.r.1989]

Bit of a delayed reply. Bit fed up with this engine now, Iv fit a vauxhall expansion bottle and left the cap ever so slightly loose to relieve pressure and all has been well! Untill after a week it's lost nearly 2.5 litres of water and antifreeze! No signs of a leak or water being blown out of expansion bottle. Gunna try go over it with a fine tooth comb to see where it's leaking. My sj gives me no trouble what so ever and runs so well, it even averages 40mpg! I really like the xud engine but damn this is testing me!