We're currently renting a house. But we are not really under contract, as we should have signed a new one beginning of October but didn't as the letting agent never send it to us. We let him know that we found a bigger house and will be moving possibly beginning of December. We didn't give him definite date yet and at the time we let him know nothing was definite yet, no contracts have been signed for new house either.
Without our consent our letting agent has put our house on the rental market, which was let within a few days and only a few viewings.
Ever since he has been asking us about a moving date.
Our new landlords don't know their moving date yet, they have bought a house and are waiting to hear back from solicitors when they can move.
Today we got a message from our letting agent, that the people who will come in our current house have given their 1 month notice and will be moving in here on 3rd of December 
Our letting agent has put our house on the rental market too early, he should have waited till we could have given him a date that is was available.
What do we do now? What happens if we need to be out of here and can't move into our new house yet?
It is so stressful and I really don't like to keep phoning our new landlords for news as I know they feel stressed about it as well, and they can't do anything about it either.