Do yourself a favour and let the garage do it. Yes it may be a faulty sensor, but more likely the fault is elsewhere. If you source your own part you will have no redress from the garage, even if they fit it. If they diagnose the fault, get the part and fit it, and charge you for the job then you will have them to go back to if the fault re-occurs. Get the part yourself and you're on your own.

Sorry, should have been clearer. The garage who diagnosed the fault will fit the sensor. They only seem to source from main dealer parts though and suggested I might be able to get the part cheaper from an online parts store. So I'm only sourcing the part, the garage diagnosed the problem and will fit it, cheers.