Thanks to racarter for directing me to this, but recording here to save anyone else the trouble of getting this installed.
A very cheap way to get automated UK mains sockets automated using a Pi and a very cheap, <£3 433mhz RF transmitter.
The code is here for the Pi
https://github.com/dmcg/raspberry-strogonanoff
Except the code for the wiringpi is now out of date and needs this fork of wiringPi-Python instead. (I tried running this on 3 Pis and spent several hours figuring this out)
https://github.com/neuralpi/WiringPi-Python
I'll add the notes on how to connect to indigo when I get it up and working (which will just be documenting racarter's good work)