My builder was against putting in a combi system as he reckoned I
would probably get leaks elsewhere due to the increased pressure
and (not unreasonably) he wasn't prepared to be responsible for
fixing existing pipework.

It's still worth doing. If increasing the pressure by one bar causes leaks,
then your system will collapse in six months anyway. If I wasn't reasonably
confident myself, I'd rather that this happened with a central heating
installer on the premises.

Christian.