Nice one PMK, very neat.
Why the high/low level detectors?
The idea was that the pump should not operate until there was enough level to ensure that the pump would prime, and would run until the level was too low to ensure water in the pump since pumps that can handle a serious influx of water often don't like running dry as Landlocked has noted.
PMK, that over-ride. I assume that it could be wired into a switcher? My choice of output transistor would have been a TIP14sommatorother, because they are easily available, pretty much bullet-proof and I can still understand how they work.
I did work out another version using a CD4001,same number of components, a bit cheaper and would have worked to lower voltages. The quad bi-lateral switch gets a bit marginal in this application on low voltages. If only I could remember where I hid it......