Hope it goes well for you - I had a subtle head gasket leak some years ago (not Hdi) and two sniff checks at different garages failed to confirm it,I didnt have time to fix myself so got a garage near where I worked to change it for me,the only evidence of a leak was a small slight 'polished' section on the HG,but it cured the cooling system pressurisation etc.
Because they were late finishing the job I didnt get the car back until the day before the car insurance ran out,so I had to drive it 70 miles to home that night,it was sort of going ok but a little down on power (xud non turbo) but the engine was sounding really weird,the car was already up for sale so I spent that saturday having to refit the cam belt - the chimps at the garage had fitted it one tooth out LOL.
Because they were late finishing the job I didnt get the car back until the day before the car insurance ran out,so I had to drive it 70 miles to home that night,it was sort of going ok but a little down on power (xud non turbo) but the engine was sounding really weird,the car was already up for sale so I spent that saturday having to refit the cam belt - the chimps at the garage had fitted it one tooth out LOL.