Two of the four tenants want to leave before contract expires

I have rented my property to two couples, and all four had signed the assured tenancy contract for one year last summer. The contract expires on 1 August 2009. however, one of the couples told me over the phone last night that they are going to separate from their marriage, and will be leaving next month. My queries:

The credit check referencing last year could not be done. I tried to do this through RLA, but their former address was not registered. However, I know the address is ok, as I went there in person to discuss the contract before they moved in. I did not take any deposit, because of the change in the law, but took two months deposit instead.

1 - Are the tenants obliged to pay me for the amount outstanding? If so, and they still leave without paying me, what can I do about it?
2 - If the other couple decide to remain, but only pay their half of the rent, is there anything I can do about this?

Suggestions/advice welcomed.