Need help quickly!

Well without seeing the actual wording of your contract - if it is a 'joint' (ie everyone is jointly and severally liable) Assured Shorthold Tenancy, then the Landlord is well within his rights to persue the remaining three tenants for any default of the rent.

Re your additional rental payments, presuming this is by Standing Order, contact your bank and stop it. Then write to the Landlord outling the amounts, the dates they were sent and to what account and ask him to return it.

He may well refuse, and I don't know where you stand on that since you do actually owe him the money, but I would imagine the Landlord should refund that money in full and then persue you through his solicitor/the courts for the remainder of the money you owe. However since you would then be saddled with the court costs, it may just be in your interest to let him take the rent arrears out of what you have paid and persue reclaiming the other tenant's 'portions' from them directly (although please realise 'portions' has no legal standing as you're probably jointly and severally liable).

It is also important to realise that the Landlord doesn't 'feel' he is owed money, he IS owed money, by you in accordance with the terms of your Agreement (presumed terms since I haven't seen the document)

What I do know is that it is very important you write the letter I described above and send it by recorded delivery, so that you have proof you tried to reclaim the money in good time, should things escalate you will need documentation like this.

A joint tenancy agreements makes all parties jointly and severally liable. To put it another way, the legally binding contract that you signed is not actually composed of individual tenants (though you see yourselves this way as individual people) but pretty much views you all as a single tenant. The landlord does not need to apportion any rent or damages to separate tenants - it is irrelevant who owes what and they can seek redress from any tenant. The assumptions you made about the arrears are also not his problem. If the rent is £x for £period according to the signed contract then that is what is owed, full stop.

Shelter offers free expert advice to tenants and will be able to advise you on where you stand with the alleged overpayments after the end of the fixed term period and how you can take the tenant to court for their failure to contribute to the rent in that period.

The LL is being quite normal, although it would have been nice for him to communicate this earlier he could reasonable have thought you were aware, as it was the responsibility of you and hte drop-out to communicate.

You can sue the drop-out to recover money if they are difficult, as in the absence of explicit agreement courts judge what the intentions of the various parties were, which was probably an equal share.