I've now bought one for the princely sum of £3.78 off eBay. On my most pessimistic forecast of 20p/kWh, I'll need to save 18.9 kWh to make it pay for itself. The model I went for was a Mercury 24 Hour timer switch (part number 350.103), also known as a MSA24 rotary switch.
After unpacking, it appeared that there were no instructions. Try as I could,
I really struggled to work out how to use it. On the right hand side was a switch (one setting was 'always on', the other was the timer), but it wasn't clear how to set it. Fortunately, by accident I accidently put some force onto the device and the black rim of the dial appeared to depress. Then it was obvious that these black outside bits need to be pushed in for when the device is to be active, and pushed out when not.
I've set the unit to turn off 3 devices (router, devolo powerline adapter and a phone charger) for 8.75 hours each night. These together consume 18.3w/hour (0.159 kWh/night), so it'll be paid for in about 4 months.
It's an experiment, as is everything here. I realise that at present I'm addressing the easy stuff, although these 3 devices are 15% of my night time usage, and are not in use.
The fridge/freezer may need to be next - I've seen some articles on a device called a Savaplug which I may need to read up on. First I need to work out exactly how good/bad the fridge is.