Assuming you have no individual meter to record consumption in your flat, then the landlord will need to apportion usage between everyone. There are a few ways in which he could do this, e.g. on a pro-rate basis based on floor space of each flat, or by the energy ratings of appliances in the flats.
He has to show that he has determined the apportioning of the bills in a fair way, and can not charge more per unit than he has actually paid.
You could have a look at
this leaflet from OFGEM which explains it all in a much clearer way than I could! It's actually guidance for resellers, but gives some good examples of how to work out the "splitting" of the bill between tenants.