The coolent light on my renault 5 campus prima lights up every day or so what do you think could be the cause?

Hi, my beloved renault 5 is well cared for ie serviced every year and looked after. I have booked it into be checked over by an aa aproved garage tomorrow as the coolent light illuminates for a few seconds every couple of days. Im just wondering if any of you have an idea of what could be the cause best and worst case scenario and how much i should expect it to cost to fix. I live in the UK. All sensible suggestions appreciated.