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 :-)