Two more small steps today:
1) A couple of variable resistors added to the circuit - one to vary the current through the IR LED and one to adjust the strength of pull-up applied to the sensor, and with a bit of tweaking I've now got readings over a useful range.
2) Readings are now being multicast onto the home network, and being received with the Linux 'emcast' tool. Saved into a file, loaded into a spreadsheet, and converted to a chart, I now have:
Next step is to take the hundred samples a second and turn it into a lower data rate feed of readings.