Ok, just had the car on the hoist at the exhaust guy's workshop. I had (wrongly) assumed I could replace the mid section and leave the first set of cats in place. Wrong. The whole assembly from in front of the first set of cats on past the secondary (centre) cat is one piece, shown if the first diagram Phil posted..
So, if I want the existing factory exhaust to be retrofittable (yes) then I need to replace the whole thing. He's saying 100 cell primary cats and delete the centre completely (replace with H pipe). He says I won't get CELs from the O2 sensors because the first cats are after the sensor.
Also, we don't have MOT or any other annual checks here, so as long as you keep paying the license fee you're fine from an emissions perspective. What say you knowledgeable lot?
Another question - what are the items marked 3 (very blurry) in the diagram on the first page of this thread? Exhaust temp sensors? Any issues with deleting them?