No warning lights come on. Just the battery symbol that you would expect when you start up. There is no other warning - the revs just drop off and there is no power.
I'm not sure if its relevant, but the car was serviced yesterday - the problem being that the garage that serviced it is on the other side of London.
Is it possible to test the sensor or will they just replace it and see if the problem is still there? I have heard they are quite expensive