I found an OPB704 kicking around and added a couple of resistors to bias it up for 5V operation.
390Ohms on the diode side biases it at about 10mA forward current from 5V supply. A 10kOhm on the phototransistor (collector) results in an almost complete rail-to-rail swing when something white is placed in front of the sensor.
Here's a screenshot of the ADC values as the 1/10ths wheel goes round on the meter:
Also just bought a light sensor module (with on-board comparator and threshold adjust) for £2 for the electricity meter sensor - perfect! Won't even need an analogue input pin for that.