Around February/March 2007, there were occurrences of fuel contamination causing upstream lambda sensor failures. These failures could (and still can!) cause the car to run erratically or even drop into safe mode.

With the fuel contamination fault, reading the codes through the OBD will return "P0134 - O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)"

Remove the rear panel by freeing the 7xT25 torx bits.

Remove the crashbar: 6x T45 torx bits.

There are 2 sensors on the later (mk4 onwards) smarts - you need to replace the one in the turbo.

Unclip the connector from the bracket on the back of the main engine wiring harness.

The connector just pulls apart.

Use a 22mm spanner to undo the lambda. It may be on quite tight with all of the heating/cooling cycles.

Remove the old lambda.

Take your replacement lambda sensor, and tighten into the turbo.

Plug the sensor back in (it will only fit one way). Note the shape of the connectors inside.

Clip back up onto the harness.

Refit the crashbar and rear panel.
Note, it takes approximately 60 miles for the lambda sensor to complete its 'readiness cycle', i.e. when it will start to register fault codes.