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MOT emissions failure - how do I fix it?

My 2001 Polo just failed MOT on emissions, high on everything. The mechanic advised I should replace the manifold cat, the standard cat and the lambda sensors. Quote for parts alone is over £700. I'd like to try and understand:

Could there be any other potential cause?
Does the parts cost sound about right?
How many Lambda sensors do I need to get?

Car just had a full service.

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Be careful over replacing all these parts,
It only takes a few minutes to check the Lambda(HO2S) sensor, and also we would check the EVAP cannister purge valve,before committing to any major replacements.
The dearest part would be the Catalytic converter,but would only change this if after all other checks had been done
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