I suppose the way to work it out is to know the temperature and the volume of hot water drawn in, then take the temperature of the cold water going into the cylinder to replace what's been drawn off and calculate how much energy it'll take to raise the cylinder temperature back to where it was at the start.

Late evening back of the envelope guesstimate...

Volume of water used: 100 litres
Initial hot water temp: 60 C
Cold water temp: 18 C
Temp difference: 42 C

Specific heat capacity of water: 4200 joules per C per kilogram (or per litre, near enough)
1 KwH = 3,600,000 joules

4200 x 42 x 100 = 17,640,000 joules = 4.9 KwH

... so about 20p duh - 35p (7p/KwH, not 4) per load to reheat the water.

I've probably overlooked something - it's late, so don't take this as gospel :-)

--
Simon