[GrahamBrown1]

Do you know what water pump was fitted? D5 head gasket failure is rare but not unheard of especially in later cars as more materiel was removed from the block where the head sits. A Cracked liner is also rare but again not unheard of.

If the issue has only come to light after the pump was changed I’d be suspicious of this. Is it a quality part? Is the impeller circulating coolant correctly at high rpm for example.

No external leaks means it’s either combustion gas entering the system to pressurise it or it’s overheating due to either poor cooling or an air lock. Could be worth checking the thermostat is operating but from your description it would appear to be ok. They do tend to bleed up ok tho and your specialist will have probably done all that anyhow.

A pressure test of the coolant system may help find the problem and prove there is an internal or external leak.