Thought I'd add this to the "Sum of human knowledge".

My 1999 1.25 fiesta zetec failed MOT for high "Lambda" (1.059 against a spec of 0.97 to 1.03). "MOT guy" suggested I fix the small leak in the end exhaust box and bring it back for retest.

So I plugged the leak and he was right - it now measures 0.997 (supposed to be 1.000)

It turns out that the "emissions" test isn't testing what the car "emits". It tests what it sucks in through small (non MOT failing) leaks.

High "Lambda", apparently, is supposed to be a sign of excessively weak mixture or incomplete combustion (high oxygen content in the exhaust gas).

So , if your car fails MOT for "Lambda" don't let some grease-monkey sell you a "Lambda Sensor" unless you know its defective.