As you have signed your contract and agreed on the tenancy start date your accommodation provider can not change this for you last minute without coming to an agreement with you. If they are trying to push the day back a few days or longer and you need to move in on time it is important you write a complaint and if it is a group of you moving in, you should all write complaints. If for some reason you have agreed for the move in date to be pushed back you should not be paying any rent for the period they are not allowing you access to the property.