Was it built before you moved in?

Did the previous home owner give permission for it to be built?

It'd be cut and dried if you had some sort of evidence that its your wall, because if it was you can do what you like with it (subject to planning permission of course). Chartered Surveyor's report confirming its 100% on your land is extremely handy though.

Were you aware of this before you bought the house? Anything in the documentation for the house suggesting an agreed boundary change?

Several thoughts.

Party wall regulations may have applied here, not sure whether they are applicable to boundary walls but worth a dig.

IF wall is entirely on your property then it is yours not theirs and you can do as you wish....

Police will not get involved unless something happens between the parties which results/could result in an offence being committed, assault or somesuch; police will regard this as a dispute proper to civil law.

Think very, very carefully before going the legal route and then think again and then decide not to....

I have experienced at close quarters two boundary dispute cases. The first entailed an argument over 4 inches of a shared driveway between two houses, went on for six years and legal costs involved were in excess of £40,000!

The second was a family member who ended up in dispute with his neighbour because he replaced some fence posts with some slightly larger ones, 100mm instead of 75mm ones. Neighbour noticed because his compost bin didn't fit back in a gap after the work because the gap was now 10mm less, first my relative knew was when he received a solicitors letter demanding reparitive works be carried out at my relatives' cost! He had a friend who was a solicitor and asked his advice. His friends reply was along the lines of do whatever you have to to make it go away but avoid going to law at almost all costs; said, 'you know that myself and the wife have brand new Mercs every year and the kids go to private schools and have ponies and horses? Well it's cases like yours could be that pay for all that!'

As others have said, boundary disputes are best avoided if possible. They are an excuse for solicitors to make money. Unfortunately the law is in practice a bit vague, because boundaries are not specified to great accuracy. If the wall is on your land, then it is tour property, and you can do what you like with it. If it remains in place for a fixed time (7 years?) with no objection from you, then the land becomes the property of your neighbour.

If a chartered surveyor has confirmed that it is completly on your side, then it would appear that you have right on your side. As your neighbour is being an ass, I don't see that you have much choice apart from informing him that it is on your property, and as such you have the right to remove it if you so wish. I assume you have showed him the survey report. He might wish to commission his own survey, and I think you should agree. You could engage someone to remove it. If he interferes, then it might become a police matter.But you must proceed in a format reasonable manner, taking steps to ensure that the neighbour is informed, and given a chance to cooperate. Keep all correspondance, and make notes of any conversations. I suspect your best approach is to simply remove the wall, once you have exhausted the negotiation route. It might pay to have a second surveyor round, just to make sure the first one got it right.

I know from personal experience that boundary disputes are a nightmare. My late mother's neighbour moved a fence while a sale was in progress. Fortunately it went through. But they were lying devious dishonest so and sos. It sounds like your neighbour might be like that. The problem with solicitors is they love sending letters to the other solicitor, and passing on replies to you, as each letter generates money.