is it not just simply the case that the land should keep an appropriate amount of the deposit?
Not really. If the house is let as a single unit, on a fixed term lease, then, until a new tenant is in place, if the tenant who left doesn't pay his portion, the other tenants become liable. This is called as being "jointly and severally liable". The vacating tenant should have assigned his portion of the lease but if he fails to do so the landlord may retain the tenant's deposit for lost rent and expenses of finding a new tenant.
The landlord is obliged to mitigate his losses as quickly as possible by finding a new tenant - but where being jointly and severally liable applies (that is, the landlord may collect the shortfall from any or all of the remaining tenants), the landlord is not at a loss; the loss (by way of having to cover the rent of the vacating tenant).
However, if the tenancy had rolled over to a Part 4, the vacating tenant does not have to find a replacement, this is the landlord's job and, this is only an opinion, the landlord may or may not be able to rely on the Jointly and severally liable aspect.
IMHO, the landlord was stupid to leave the utility bills in his name. They are usually put in the name of one of the tenants, each tenant being responsible for one utility bill and collects the share from each tenant while one tenant takes responsibility for paying the full rent.