Hi Mike - I've used JG Speedfit (from Screwfix) occasionally but I have to
say I prefer not to use pushfit where I can't get to it easily to check.
Probably a bit irrational I know. Also check whatever you use is actually
suited for central heating where you typically have chemical additives as
well. I'm pretty sure most are.
As an aside, when I had a small washer hose leak problem in our kitchen
hidden beneath cupboards some while back, I turned up some little water leak
detectors on e-bay for a few quid each. These are just little battery
powered boxes with a couple of contacts on them which alarm if they come
into contact with water. For the ones I got, the contact plate is removable
and extends out on a wire so the alarm itself can be in a cupboard, and the
contact plate on the floor below weighted down with something so it won't
move, but I haven't been able to find those since. However, there is a
similar item to mine on e-bay currently on lot 140483711394 (search on
"water leak detector" for other products) which is ostensibly for overflow
but could probably be adapted if that contact plate isn't suitable.
I have them behind my bath and shower tray panels too and in my airing
cupboard where my c/h pipes, pump and shower pump is - and just change them
when I swap my smoke alarm batteries every year or so. Bit obsesive maybe
but the unseen dripping of water from that hose leak became a bigger and
expensive problem as they often do!