If you're in a house share, each person in the house needs to pay a share of the water bill. In 2006-07, the average household water bill was £294 for a year. To calculate your monthly contribution, divide your annual water bill by 12 then divide again by the number of people in your house (including you!)
Egs: If you live with 3 other people and your water bill is £300, then you will pay £6.25 each month.
It may be a good idea to pay the bill in instalments if you can, so you don't have to pay it all at once. IE: you could pay in two halves over the year, or pay by direct debit each month.