Discovery 4 HSE TD7 - Brake Discs

I have a June 2012 Discovery 4 HSE which was serviced back in July (24,000) by a main LR dealer at which point the health check mentioned that the front brake pads were lipped and the back discs were 60% worn. They recommended a follow up at the end of Sep 2012. I have since been back at the beginning of October and paid £987 for the replacement of both front and rear pads & discs as they have said that the rear were worn down to the metal and the front were lipped.
My main concern is why didnt the sensors work - the dealer said that the pads have worn unevenly and therefore as the sensor is only on one side it didnt go off but they have charged me for replacing them!!!?, is this cost excessive?, do I have any warranty claims as I've owned LRs for over 15 years and never had to replace the pads let alone discs within the first 3 years?