You can turn the light off by disconnecting the battery for about 30 minutes, then reconnecting. This will reset the car's ECU. If it is the oxygen sensor, (also called an O2 or Lambda sensor), it will soon come on due to re-sensing a fault. Replace the sensor. Try looking online at Lambdapower.co.uk for help and advice, and also try a company called Motor Parts Direct, for the parts and prices. I highly recommend the removal tool for doing this; it's less than £10 for the tool and makes it so much easier.
Howard, May 2011