Well, it turned out that the "some chassis bit in the boot floor" has a bracket inside it to strengthen it, it's up to the owner to fit the captive nut & restraint bolt hardware... At least the hole is there.
Interesting point, is that the restraint point is supposed to be directly in the centre of a seating position (as required by law here), but the alfa provisions are about 4 inches off the centre line, meaning the restraint won't be straight...
Anyway, the manual says...