In situations like these I've always dealt with utlities split equally between everyone.
Rent is split on the basis that the room being shared pays more overall, with the extra amount to reflect the additional tenant's usage of the communal areas and the extra pressure/inconvenience this places on the remaining tenants (things like less space in communal areas, less fridge space, bathroom usage, things like that).
I think we had it so there was 4 of us paying:
£350
£350
£350
£300
When the 5th person moved in, it went to
£400 (shared room)
£335
£335
£280
Or there and thereabouts.