Dan,
There is a valve in the line thats supposed to stop that on some cars, but iirc the Clio relies on the pump to provide a sort of hydraulic seal.
If the pump is worn, you may also notice it takes a moment from pushing the button till the fluid sprays.
One pump does both front and rear screens by running in two directions, one way for front other way for rear, so in theory if the pumps worn, the "fault" should be at both ends of the car