Just to let anyone interested know.....
I took the Windshield Washer Level Sensor apart to discover that a 180 Ohm resistor had corroded through due to water ingress. Simple enough job to replace the resistor and reassemble the sensor ensuring it was watertight. Low coolant light now out and problem fixed! Both sensors now work as they should.
Many thanks to M.B.DOC for the help given. Never would have worked it out without that!