Hi,
if you want to charge fair utility bills then you could get whats called pre paid meters and timers which are cheap and can be installed by an elctrision, please note i am not on about asking you eletric company to change your meter to pre paid just install your own, this can be handy as if you have bedsits the tenent buys cards off you from £1, £5, £10 which are provided by RDL http://www.rdlproducts.com/php/show-product.php?p=103
which can be found online though some peopel just by refurbished ones becouse there cheaper, though in a shard house a timer on items like hair dryers, irons and washing machines, TVs can help lower the bill and can be removed when house is sold on or you decide to move back there and take 20p, 50p and £1 and you set the time limit yourself, http://www.electricmeters.co.uk/prepaid_services.htm and http://www.electricmeters.co.uk/prepaid_timers.htm, have a look though the links might be helpfull but could stop arguments with tenants becouse thay dont have to use thouse services and could make life eser for you.