Spotted a plug-in 'electricity usage datalogger' at B&Q, and since this is something I've wanted to do for a while I stumped up the £25 required and bought one.
It's basically a 13amp adaptor with an LCD display which you plug in between a 13amp socket and an appliance. It then records how many amps the appliance draws when it's switched on. You can enter price per KWh (limitation: it doesn't have enough decimal places to cope with the £0.00902 that our electricity costs) and it works out how much you've spent over time.
I've had it plugged into our frost-free fridge-freezer for a week, starting from a fully warm and 'defrosted' state because the fridge thermostat blew and needed replacement. It's therefore provided a reasonable report on how much juice it needs to cool down to operating temperature and then maintain it.
Turns out to be £1.18 per week, which is roughly 1.87 KWh per day. Not too bad.
It's now plugged into my mobile phone charger, to see if it actually uses and power at all when a phone's not connected up.