coolant warning light

I have a skoda fabia 1.4 classic petrol x reg

Well i see other people had problems with their red warning light and i have looked at many posts but have got knowhere fast!

The problem is ...I start the car and lights act as they should.....then after a minute or two the coolant warning light beeps and flashes and stays on........no other problems temp gauge fine, no over heating, have driven it for miles with no problem. The questions i have are as follows.

1 Ive been told that it could be a choice of 2 sensors that are at fault...i would have thought temp gauge sensor is ok as temp gauge is ok

2 skoda garage want to charge me nearly 80 quid for testing b4 starting and say it takes 2 hours, is this reasonable? Hope you can help!