1996 Mk3 Golf Cabriolet. 2.0l 8V AGG Engine

Failed the MOT emissions test miserably apparently. Just wondering where the best place is to start as the garage seem to be a bit clueless.

Excessive Carbon Monoxide content at idle
Excessive Carbon Monoxide content at fast idle
Lambda sensor reading outside specified limits
Excessive hydrocarbon content after 2nd fast idle.