Oxygen sensor problems

Hello all.

I have the joy of an upcoming NCT ( Irish MOT ), and am having a problem with an engine management warning light on a Mk 1 Octavia ( with 80k miles ). The diagnostics pointed to a problem with an Oxygen sensor. My mechanic ( ex VAG ) told me that the problem was usually with the breather pipes rather than the sensor itself so that was where to start. Having been relieved of substantial amounts of "folding", the warning light popped on within a couple of hundred metres of leaving the garage. The prognosis is that the Oxygen Sensor ( front ) needs to be changed.

Am I being had? Or is it a sensible thing to replace the Breather pipes first ?

Thanks everyone