How Green is Electricity?

The amount of carbon produced per kWh of UK electricity varies considerably depending on demand and the availablity of wind and other renewables. It can be hard to calculate precisely, and it is arguable whether one should worry about the average figure at any point, or the figure for the generation which would be used to produce any further demand. In other words, if the country is currently running entirely on wind (unlikely), but switching this light-bulb on causes a coal-fired power-station to be started (unlikely), do I consider the light-bulb to be using electricity costing close to 1kg CO2 per kWh, as the extra load caused the coal-fired station to be fired up, or do I take an average figure which will be almost zero?

Anyway, various pages with real-time estimates below.