My car failed its Emissions test can you help?

After paying out £160 for brake shoes, wheel cylinder & exhaust back box my 52 reg citreon saxo 1.1cc failed its mot emission test. About one week before the test I added a petrol additive to the fuel tank & on the day of the test I drove 2miles to warm the engine.
The fail certificate states
Fast idle test
RPM not checked
CO 0.83%
HC 286PPM
Lambda 1.04

Second fast idle test
RPM not checked
CO 1.06%
HC 409PPM
Lambda 1.04

Natural Idle test
RPM not checked
CO 0.96%
I spoke to the tester & asked whether the petrol additive caused the fail & he said he didn't think so & added that the diagnostic meter said the engine was misfiring but when he checked this it wasn't, he told me to take it to an auto-electrician but after I've spent so much for repairs I'd rather not, anyone got any ideas ?,P.S. The lamba unit was changed at the last Mot.only 4k miles ago..