Economy Energy charges around 16p per kWh on a pre-payment meter. (It's 15.78p around here but prices vary slightly across the country).
With such a charge, a modern efficient fridge-freezer costs around 20p per day to run but an older model might cost twice that. So (assuming that you've got an older, inefficient fridge-freezer) perhaps 15p of electricity might have been used in simply keeping your food cool between 0900 (when you had £5 credit) and nearly 1800 (when you checked your meter).
Have you got anything else plugged in? A TV on standby (rather than unplugged) might have used 1p in electricity during that time. Other devices (such as printers) continue to use small amounts of electricity when they appear to be switched off. (If you don't believe me, just place a portable radio next to an 'off' printer and listen to the interference!).
What type of light bulbs are you using? If they're the older tungsten type then 3 x 100W for nearly 3 hours would use about 14p in electricity.