I've now bolted 3 M10 exhaust clamps over where they pipe cut it, at the upstream end. Doubled up on the nuts. So a pipecutter won't work anymore. I also put two nuts, tightened together, on the open end of one of the two spring bolts to the manifold. So if they ever tried to unbolt it (frankly I doubt they know how to undo a nut) then they will need 2 spanners not just one impact wrench, and a lot of fiddling about.
I tippexed 'LOOK' and an arrow on the cat so they see my efforts before they cut the downstream end.
It's still vulnerable to a reciprocating saw so I've ordered some more clamps and some hardened steel cable, I will loop that tight under all the clamps .
Hopefully even if it takes them a tenth as long to remove this as it took me to install it will put them off and they'll seek an easier target, maybe a Prius.