Well I got the car booked in tomorrow for them to look at it and see whats what. As soon as these beeps and lights come on i tend to have a look over everything just in case. The coolant level hasn't dropped and the levels are clearly marked on the expansion tank and the gauge rises as normal. There is no difference if the car temp when sitting in traffic etc so hopefully its just a sensor. Will let you guys know the results
I had my bottle replaced at the last service (sensor is part of the bottle) and it was only about £8 + VAT. As it only took about 5 minutes to fit the new bottle & top up the coolant, there was no labour charge.