The job's done and I had a wonderful drive home as if in a different car. It is astonishingly quiet, so much so that I can hear the mechanical sounds of the engine spinning rather than just exhaust roar. The combination of low noise and swift acceleration gives the impression that the car is even faster. Perhaps it is, knowing that there is no exhaust restriction at all. Ian at H&S explained to me that both the centre silencer and the two back boxes are stuffed but feature a straight-through design, so there is probably less impediment to gas flow. The two Helmholz resonators are empty, as they should be.
External noise at idle is the same as stock but you get a nice subdued tone at revs.
Without knowing whose exhaust came off, he did say the back box is a bad design and the overall workmanship with all the welds (and the cracking) pretty poor. I decided not to try selling it to a poor unsuspecting buyer so I had Ian scrap it.