Many travails in attempts to reduce power requirements so that devices can run on disposable battery for a couple of years. The boards I was using from 2 major manufacturers do not seem to be very well optimised for low power during sleep use. But the Rocket Scream (RS) devices turn out to be just the thing.
However, running sensors from the 3.3V pins on an Arduino means that the sensors continue to draw power (mA) even though the board is asleep (uA). Tried to power the sensors from a digital pin, but these can only deliver 7mA and sensors require 8! Advised by the very helpful RS proprietor to use a MOSFET. Out of my electronics depth, but after a bit of study, bingo. The MOSFET acts as a switch, powered by a digital pin and delivering the full current to the sensor. And only costs 70c.