I forgot to say that of course, this is the Rolls Royce system, and does
cost a bit more.  However, since repairing the leaks costs a lot more, there
is little point for me in cutting the corners.

Loose laid sheet membranes are the worst.  Water can track from anywhere
before becoming clear there is a leak.  At least with bonded systems you
have a chance of repairing them (which is another reason why suspended
paving is good).

Cheers

david