Many thanks for all your help. The full story is as follows: Took the car to the Vauxhall garage as my usual repairer did not have a diagnostic computer which would be needed to set the sensors. Called into the Vaux garage on a Saturday morning, explained the problem, and the nice man brought up the part on his computer screen. The part was £80 plus vat, labour, should get change from £200, but the technical manager would be in on Monday and he would ring with a more accurate quote. This he did, and this is where the £280 comes from. I agreed to this over the phone and arranged for the Omega to go in on Wednesday. On collection I was informed that Vauxhall had sent the wrong part and booked the car in for Friday. Arrived late afternoon and this is when I was given the bill for £804.76. That evening I wrote to the Managing Director complaining (recorded delivery) and eventually received a letter from their Retail Operator admitting that there had been a misunderstanding internally about which part had been ordered. He even admits, and I quote: "I fully understand your astonishment when you collected the car, it was the same for me as I understood you knew the final price" unquote. At no time did I give permission for Downside Motors to carry out any work which would amount to more than £280.
I contacted Vauxhall UK M.D. who put me onto his Executive Support Team. After various communications the bottom line was, as this is not a manufacturing fault we cannot help in this matter.
I stress again that at no time did I authorise any other work to be carried out. I did not receive any communication from Downside informing me of the cost.
The component that was replaced is a Aircon Control Unit. V0009196979. Regulator-Electron.
So there you go folks, as has been said, make sure everything is in writing before going ahead with any repairs. It's a difficult situation 'cos the heating system woks fine now!
Thanks to all of you who have replied, and thanks for your comments and suggestions, some of which I shall explore. Happy Christmas and Best Wishes for the New Year. Regards, Arc.