Not sure what pressure a blanking peg with grab ring removed (as per instructions) is okay too, but it's apparently less than the pressure generated in mains water system at the point where I turn the hosepipe off, the wife screams, and I dance through the kitchen with the grace of an overweight hippo wearing muddy trainers, all over my newly laid tiles, lunging for the lever valve I thankfully fitted just upstream of all my temporary hook-ups for the kitchen sink and outside tap.
Blanking plug now refitted with grab ring installed this time. I'll never get it out, but at least it won't come out either.
Ben