@AndrewHead, sure, lots more details, but was looking for interest first. Good questions, thanks.
flow sensor; a hall-effect fluid flow sensor, is wired to pin D2 and used with
attachInterrupt pin change interrupts; on the end of the pipe where it reaches the storage tank; actual sensor was a Jaycar one that is no longer listed as far as I can see, but Little Bird has several alternatives, such as the half inch one from Adafruit,
full sensor; a side of tank float switch, wired to an input pin, which has proven to be a great place for frogs to take a rest from swimming, hence the ...
overflow sensor; two bare aluminium bars screwed into a terminal block mounted under the overflow vent; chosen so the sensor lasts a fair while despite occasional electrolysis.
Also, forgot to mention, the purpose of the vent next to the pump is so that static water pressure in the long pipeline can be released during the self-priming phase of pumping. Pump is easily able to self-prime, but only if there's no pressure on the output. So the sketch opens the vent solenoid valve, starts the pump, waits a few minutes, and then closes the vent. The priming time is a parameter that can be changed from the house.
Electronics for both sites are mounted in the base of a 3m section of 45mm plastic tubing, which contains a ribbon antenna cut and tuned for 151 MHz. Tubing is capped at the top, and strapped to a steel fence post using wire ties. A waterproof vent is made in the top to keep the temperature of the circuit board reasonable in full sun, otherwise the radio performance degrades. Air flow occurs due to chimney effect of solar heating, with no fan required.