Just writing out my findings in a seperate thread so that the results can be found easily by future sufferers of apparent sensor failure in the hope that it might save someone a few (hundred?) quid.
My SOSkoda's symptoms were that it did not like starting when warm and would rev up to ~2500rpm all by itself when being driven. I suspected a faulty sensor but as there's quite a few of them, and the cost of new ones was more than the car was worth, it just sat on the driveway, looking sorry for itself.
Thanks to this group, and BRISkoda, I started taking things apart and found that the lambda sensor wasn't working as it should. It's supposed to generate a voltage between 0.1v and 0.9v from cold to hot respectively, which falls away quickly when the heat is removed. I unscrewed the sensor from the exhaust, clamped it in a vice, connected a multimeter, and heated the tip with a blowtorch. The voltage *eventually* rose to 0.8v and slowly fell away to -0.2v (minus!). After trying this a few times I noticed that the results were improving so I played the torch all around the tip of the sensor for a few minutes, especially into the three vents, and tested again. This time the voltage shot straight up to a steady 0.9v and fell very quickly to a steady 0.1v when the heat was removed, which is exactly what it's supposed to do.
I was advised that the water temperature sensor is a common failure point so removed it to test. NB This is the sensor for the ECU, commonly placed in the inlet manifold, not the sensor for the interior temp. gauge. Dunking it in hot water showed it was flashing a resistance for a split second then showing open circuit at any temperature so a new one was bought from the very nice chaps at Evans Motor Factors in Wales ( snipped-for-privacy@btconnect.com), after a search on eBay, for £10.50. I tested the new sensor and it showed a steady resistance.
The car now runs properly and seems back to its normal, horribly basic, noisy, but incredibly useful, self.
So, instead of a 'mechanic' telling me I need a new cat, lambda sensor and temperature sensor and charging me loads, it was all cured for £10.50.
Definitely worth a try as the whole job was quite simple - I just didn't know what results I was looking for until I was told.
Si