There was just enough room to do this with the dremel extension:
Then a slot in the silencer so the clamp can squeeze it:
... and the GBS bracket which bolts into the floorpan with 3x M10 bolts/nylocs etc. It turned out I couldn't push the silencer onto the manifold with the bracket in place, so it had to come off again.
Silencer fitted - pipe clip & a smear of exhaust paste at the front...
Spring clip with an offcut of exhaust wrap at the rear....
It doesn't sound much quieter on the video - but it is - theres still some oompf there but its a gentler, rounder sound. The video is grainy because it was dark & low light in the garage when I took this:
I ran the engine long enough to get the heating up to temp, also for the first time had hot water through the cabin heater, I also ran the two new GBS maps for power and emissions; I couldn't immediately work out how to use the Lambda sensor - but had open loop readings coming through so its all set to have a play another day.
Everything worked fine except - my leak returned on the low pressure fuel pump - in the same place - the brass fitting on the outlet side of the pump - it will have to wait until I have daylight to fix.