I was speaking to a customer of a local "Authorised Dealer" (Motor Codes Subsciber) who had taken his car in because his mil lamp came on.

They "scanned it" and replaced the 02 sensor, (£140) they also told him it had a "split vacuum/air pipe" but didn't have the pipe in stock, so they let him take the vehicle overnight on the understanding that he returned next day to finish repair and to pay. (longstanding customer !!)
Whilst he had the vehicle overnight the mil came on again. He returned it to dealer, they replaced the pipe, then gave him a bill for £300.
He questioned whether it actually needed the 02 sensor in the first place as the mil had come back on.
They told him that the "diagnostic tool" had said it was a faulty 02 and if he didn't pay, he wasn't getting his car back.

I've advised him to ask for the old parts, and take the first step in advising motor codes that he is unhappy. (Be interesting to see how this pans out)
I suspect the 02 sensor was merely doing its job and was stuck lean due to the excess air (split pipe)
His bill should have been less than £70 even at dealer prices for a diag and a new hose.

Good to see that Motor Codes are doing there job and only allowing the good garages to sign up, the ones the public can put there trust in.
Good job Chris !!

Regards
Dave