To add to what others have said – some neighbours engaged in boundary disputes have spent hundreds of thousands on legal fees – not necessarily being successful.
I have the land registry plots for my house & garden, which are supposed to show the ‘legal’ boundary of the property. Reproduced A4 size, the boundary line is over 10mm thick – it would be no exaggeration to say that at life size, the thickness of this line would be well over 1 metre. In theory, the legal boundary could lie anywhere within this 1 metre line.
My house is an end terrace – in reality, I would expect a court to up-hold the location of the boundary fence on three sides of my property. With regards the precise location of the boundary between my neighbours property, is it somewhat easier than some to determine this, since the property is a staggered terrace – and one need only extend the brick line of the house to locate the precise division.
That said, should either side relocate the dividing fence – then after a number of years, this would legally become the boundary, indicating who owns what space/land.
Therefore as others have said, you need to get legal resolution of this matter – but it is by no means a simple task.