[GMcL]

I've got the same, earlier, engine as what you have, mine came with 163PS I've been running it for the last 8 years with 195PS and 470Nm. When cold there is no pressure in the system, no surprises there.

I've been told the rads can leak down in the bottom corner, which is what Clan & Kev have also written, you'd be unlucky if it was the HG. Not impossible but very rare. A sniffer test would tell you if any gases were getting into the coolant as would bubbles popping up through the expansion tank or mayo in the oil filler as others have suggested.

The garage didn't replace the coolant prior to you taking delivery did they ?

Keep us posted how you get on