I moved out of a property over 2 years ago, I have been asking my ex to take me off the mortgage all this time to which he has done nothing, I now find out through a third party he is moving someone in, I have now decided to sell, but he is trying to stop me getting in, so I have taken back door key to allow estate agents in to value it is this OK

Are you on the deed? If you are not on the deed you have nothing to sell. If you are, then you have to file suit to partition the property to force a sale.

Your name on the loan gives you no right what so ever. If your name is not on the deed you have no rights to go in the house, no right to have a key and no rights to sell the house.