Help - failed MOT on emissions plus error codes on computer

[Adam32]

Despite putting some cleaner though the vehicle, my 2001 Saxo Furio has just failed the MOT on emissions for the Lambda reading on 2nd fast idle. The reading was 1.105 when the maximum limit is 1.030. On the first fast idle test the CO was also high at 0.309 when the maximum limit is 0.300. The MOT tester suggested it could be a faulty Lambda sensor - what do people think?

One other thing to note is I sometimes get the engine management light come on the dash and the car seems to stutter badly. This is an intermittent problem as the last few weeks everything has been okay. When the problem occurred before I borrowed a neighbours computer and got the following two errors:

P0420 - Catalyst system efficiency below threshold bank 1
P0300 - Random / Multiple Cylinder Misfire detected

Do you think these problems are related to the emissions fail (although no lights were on the dash and the car was not stuttering during MOT test.)

Where should I go from here, do you think the problem is a faulty Lambda sensor? Where should I start? The problem is money is tight at the moment, so if I can't fix this myself and get it through the MOT I will have to scrap the vehicle.