Coolant message

[Johndoh]

I have 2012 2.0,estate diesel I have an issue with the cooling system, message on dash when I turn engine on coolant low top up, well done this, still there checked for leaks looking good, checked float and put new one in still there, but the thing is the coolant level as not gone down since I topped it up to the level of mark on bottle and car temp seems to be OK comes up to running temp in around 5 min but never go's over 90 on clock any help on this would be great as I need to use the car for weekly hospital appointments