Hello again!
I've just taken a look in the Argos catalogue (which I find is an invaluable source of information for answering questions on AB). Many security lights are rated at 500W. That means that they'll use 0.50 kWh of electricity for every hour that they're turned on. (So work out how many hours the light is on for and add your results to my previous figures).
4 x 40W bulbs will use 0.16 kWh for every hour that they're switched on. So, if that lighting unit is on for 3 hours, you'll use 0.48 kWh.
As Ian says, a 40W bulb of one type will use the same amount of electricity as any other 40W bulb. The only difference is in their efficiency in converting electricity to light. A 40W tungsten bulb, for example, will only produce enough light for a desk lamp or a fairly dim bedside lamp. A 40W 'low energy' bulb will light quite a large room, fairly brightly. They both use the same amount of electricity but a tungsten bulb converts much of the electricity to heat, whereas the low-energy bulb converts far more of it to light.
I certainly think that you need to consider switching to low energy bulbs. (They're now far cheaper than they were a year or two. Try Morrisons or Tesco). However, I still suspect that your chest freezer might be responsible for using a lot of electricity. Ensure that it's properly defrosted, keep it well-stocked (because it will use less electricity) and invest in a thermometer:
http://www.choiceful.com/choiceful-id-20107-Fr idgeFreezer-Thermometer.html
Use it in both your chest freezer and your fridge-freezer to check that the temperatures aren't lower than necessary. (Aim for 4C in the fridge and -18C in the freezers). If the temperatures are lower, turn the thermostat back to save some electricity.
Chris