Jonathan
Personally I think you are making life difficult for yourself, due to the reasons you have questioned yourself. Your intentions are fantastic, by only wanting to pass on the actual cost to your tenants, but in practicality it will only end up with confusion when you have voids in the other rooms, and potentially conflict between tenants.
Have you thought about charging your rent to include utilities, so there is no way of any confusion or fluctuating bills for the tenants. This is common with multi-let/HMO situations, you know the typical running costs of the house, as you have the historical info, so you can easily calculate this per room per month (using current energy costs). Build in a cushion for voids (based on your current occupancy rates), and any other factors which you may feel necessary to take account of, then set a monthly rate for utilities and council tax, which is added to the rental figure for your per room per month rate. Get it right and you cover all your running costs and your tenants have a fixed monthly charge, irrelevant whether other rooms in the house are occupied or not.
Charging current tenants for voids, on your basis is wholly unfair in my opinion, as this is your problem not theirs. Simplicity is best, and will ensure that you don't lose out and your tenants know their monthly costs in advance.
I have PM'd you with a solution for you to consider.
Regards
Jeremy
