OK - so I topped up the coolant at the beginning of lunch today - it was quite low to be honest.
I drove to the garage, washed the car, drove back and on the way back the coolant low warning came on again. I checked under the bonnet, and yep, the level was low again.
I topped back up with water again, and as yet it hasn't gone down - albeit the car hasnt been running, so I guess the acid test will be when it gets used again.
This doesnt sound great to me, and I'm very much a novice...