Headgasket blown of 146 Turbo Diesel

[iceking]

I have a 146 Turbo Diesel from 1996 230Km.
For some time the coolwater temp was high especially at hot days. However the temp when high and engine had no cooling water anymore! car was stopped immediately.
I have changed the thermostat en radiator.
Temp is normal. However, there seems to more pressure than normal in the cooling system and it it loosing coolant after driving at normal speed ( 120 Km/h).
I suspect the headgasket to be blown.. How can I check this?
How difficult/easy is it to change the gasket? I don't have any special Alfa Romeo tools (yet).
Are there any pointer on the web to this subject?

greetings Ernst