Cant see another way round it hes gonna have to bite the bullet and buy the 2m of 22mm!
Op you could always wrap the olive with copious amounts of ptfe.
Would normally be run in copper. The fact that it is dripping could mean a few things.
Installer didnt make a proper allowance for expansion, he has made proper allowance but it has failed or the relief valve is faulty.
As above its not just a matter of screwing a bayonet fitting in and leaving.
Numerous tests have to be done to ensure your system is safe.
how much do you expect to pay?
85.00 pound installation charge sounds fair to me.