Disco 2 v8 poss head gasket troubles

Hi all
I have recently bought a 2000 disco 2 v8 and love everything about it apart from it drives me potty!!!! I have always been a defender man(all diesels) and find the v8 out of my comfort zone.
Today on a typical chilly July morn I put my heater on expecting wonderful things with it being a petrol and low and behold it was rubbish! Slightly better when engine revving but still poor, so I contacted a friend who is a landrover mechanic and after ruling out several things he pointed to the head gasket, mainly due to the top radiator hose being heavily pressured.
I have no doubts he is not wrong as he never is, but I am not loosing any coolant, no oil in water or vice versa it this normal?
Also any idea of costs to have them replaced?
Sorry for such a long post but wanted to cover all bases

Thermostat stuck open maybe which is awkward to access on d2. Check pressure release expansion cap. Bleed air out of top hose.
Does coolant temp warm up quickly enough and does it stay in the normal without any fluctuation?

As above I had hg failure for almost a year with no symptoms apart from loss of coolant over time and pressurised top hose eventually leading to water leaks.
As to cost cheap if you diy its got to be almost the simplest hg todo ever

Don't rip it apart for a head job until you get the coolant checked for exhaust gasses, from there is a simple diagnosis.