As the other posters have pointed out, you should not be expected to pay for supply before you entered the property. i can only go on another supplier's rules, but when it came down to it all you should have to do is supply a copy of your tenancy/lease agreement, which will have the date you moved in.

Unfortunately, if you don't have a start read from when you entered the property, they may have to use an estimated start read to bill you from. Any usage from the date you moved in, im afraid, is up to the supplier really. It's not their responsibility to ensure that the bill is split fairly between your flatmates, only that you are billed for the correct amounts you are liable for.