okay, thanks for the feedback.

i've come to the conclusion that i've followed bad advice which is
probably the cause of the problem. i'd also been warned not to
overtighten compression joints, but advice here seems to suggest that
isn't possible.

anyway, i think i'll replace some of my compression joints with push
fittings in the hope of avoiding a repeat disaster (although i've got a
push fit joint leaking slightly at the moment as well !!).