Topic

It could be.

Open the joint and smear some exhaust paste in there.

So how did this story end up for Robmj52..? I have a similar problem. Here are my values from the MOT:

CO at 2500 rpm: 0,3 (approved)
CO: 0,2 (approved)
Lambda: 1,46 (! not approved, limit is 1,03)
HC: 541 (! not approved, limit is 200)

I wouldn't think that other than the catalyst the rear part of the exhaust system would have any meaningful impact on emissions?. Surely this is a (over) fueling or ignition issue?

Aftermarket cats are notorious for passing 1st year after fitting and possibly second year, but failing after that. This is due to the minimum amount of reactive component within it.
They also usually require to be very hot to pass, again for the same reason. Try to get the tester to do the emissions test immediately after you have given the car a good run.
I went through this almost every year until I fitted a standard MGR type approved cat. The car has flown through the emissions every year since.

Thanks guys