Encroachment

[ZillyG]

My garden boundary is a bit abnormal, but previously my neighbour and I have agreed in writing (with some photos) as to where the boundary line lies. My solicitor also has a copy of the agreed line for the boundary.
My neighbour has now put items over the agreed boundary line (tubs, trellis, wooden edging etc). They have refused to remove the items.
I know I can prove they are over the boundary and they are eroding the agreed boundary line.
Can I legally move these items? I would be placing them back on their land, of course.
I know it sounds petty, but I want to maintain the boundary as clear as possible as I might be moving in a few years time.

[des8]

did you register the agreed boundary at land registry?

[ZillyG]

did you register the agreed boundary at land registry?

No, I didn't know I had to/could do.

The plan I have from my deeds is right, it's just on the ground it was a bit muddled. If it deviates, then it's only by a tiny amount. I'm not bothered by that.
The neighbour themselves have always told me where the boundary line is, I had agreed to that, but now they're changing it. This is what I'm bothered by.
They've previously moved a fence because it was very clearly over the boundary, but now they say they're not interested.