on first 'short' mot test lambda was just over at 1.032 other readings ok. when did full test first fast idle was a fail on co but pass on lambda, while second fast idle was ok on co but fail on lambda.
I had something similar on one of my other cars. Second fast idle passing on CO *could* be just that the cats needed to get hot before operating at full efficiency. The Lambda fail could be an air leak on the new exhaust.
Maybe when things get hot enough for the cat to work the exhaust is opening up an expansion leak at a join somewhere ?
My experience on the TVR was a massive fail on CO , with lambda right on the limit from cold. Took it for a spin, came back and kept revs up to keep things toasty , passed CO with flying colours (cat now hot) but lambda went over the edge. Quick look under the bonnet showed a small leak where the manifolds joined the exhaust. 2 minutes with the spanners, leak solved and all passed.
I cant say for sure that this is your problem but I would say its worth going over all the exhaust joints again. My experience with the TVR the leak was after the lambda sensor, so no faults shown on the ECU , just too much air in the system.