Just as Dbug said. W Grose also said that to claim on the warranty I had to take the car to them and pay £40 for half an hour of labour for them to check the sensor on the car before deciding whether to fit a new one under the "guarantee". They say they cannot check a sensor off the car. I say, they don't need to check the sensor, this is a known problem, I've described the symptoms and given them the fault code and I should just be believed and refunded.
Essentially, this means there is no guarantee because it would cost at least £50 to get someone to tow the car to the dealer. As far as I am concerned it's a neat and deliberate get-out of servicing the warranty on a part well-known to be unreliable.
So, I just bought a new one, under protest, but I think this is a matter for trading standards. I am going to write to the guv'nor at W Grose and give them a chance to refund me first and if not, I'm going to war.