Had MOT re-test today, everything was fine except for emissions again! Failed on CO and LAMBDA..

I changed the Lambda Sensor and Cat in an effort to lower the emissions first time round.
Also got rid of all the exhaust blows

All I saw was a slight reduction in lambda (it only just failed), CO didn't change but if anything it increased.

I know there is a chance that the different Cat and Lambda probe may be dud aswell so I need to check them, can anyone give any advice on what might have gone wrong?

When fixing the downpipe to manifold, I didn't use a new gasket (used one which was only 10k miles old). Could this cause an air leak which would mess up the Lambda reading, even if though it is past the sensor?

Also, is it likely to be my temp sender, or anything on the air intake side?

I can post up figures if it will help, really need some guidance on this though please guys.