If running cost is an issue you can power them through a 'frost stat', that's a thermostat controlled socket that is used in greenhouses to turn heating on when there is a danger of frost. They are usually set at 5 degrees C, but often are adjustable.
Doing this will cut the running cost down considerably
20 units per week absolute max (60W * 2 * 24 hours * 7 days = 20.16kWh) (which makes it about 1 pound per week at 5p per kwh plus whatever the admin / standing fee is).
It will actually be much less than this as some of them have a 'stat which will turn it on and off as required.
Two in the Force 4 catalogue which came with the Nov. PBO.
600 mm / 80 watt @ £29.95 and 900mm / 135 watt @ £34.95
I'm sure I've seen them in the Screwfix catalogue in the past. It might be worth looking at their website.
Electrical suppliers to the trade will have them but not the discount electrical superstores. May also be available from garden suppliers who specialise in greenhouse accessories but perhaps only in larger sizes