Ok - actually you did need permission to change the fence but the neighbour could not have refused under the Access to Neighbouring Land Act 1992.
As such you are now entitled to place a new fence on the boundary. If there is no doubt that this is indeed the true boundary they can not stop you.
If they try then you can get a Solicitor to write and set out your position. That is the boundary is true, you are entitled to put a fence there and they have to let you under the above Act. If they refuse then your Solicitor can say you will go to Court.
If they still refuse then you can issue proceedings using form N1:
The Court will set the matter down and decide whether to make an order making them let you.
If the Court does and the neighbour refuses, this could be contempt of Court which they may be fined, warned or sent to prison.
Can I clarify anything for you about this today please?