Slight update to the thread as I did some more work.
She failed her MOT on emissions. I took the old O2 sensor out for a look, and it had fallen to bits, so I ordered a proper Bosch one to replace it:
Tried again with a tank full of emissions cleaner/reducer, and whilst this sorted out the issue of failing the HC (hydrocarbon) emissions, the CO was still a mile out (down to 1.3% from 6%, but needed to be 0.3%).
Ended up relenting and finally buying a system to replace the old one, however, I didn't want to go through the faff of having the downpipes modified to clear the ARB and steering column again, so some chopping was required:
Rather than cut + weld, a local fab shop cut them for me, and then sleeved the new system:
This makes removal/refitting a much easier job!
I cut some slits in the sleeve with an angle grinder, bought a cheap clamp from Halfrauds, and voila:
First attempt:
Second attempt with new O2 sensor + tank full of emissions cleaner:
Final attempt with the new system:
Road legal for another year!