Tyre Pressure monitoring system

I keep getting a fault indicator stating that the Tyre Pressures are not being monitored, it will clear off the screen very quickly, but remains in the fault log you can view from within the car until you switch off when it clears completely. Citroen Dealer have looked and found no fault with sensors or fault codes stored. So cannot offer a solution. Other than having them turned off completely I do not know what else to do, but the beeping once (sometimes twice) during each journey is annoying.
Anyone seen this before or have any solutions to offer?