Neighbour attached their fence to mine.

Any dispute over boundries with neighbours that involve any legal representation, the only winner will be the solicitors.

"Last year I put a new fence up in front of the fence we share with a neighbour. The holes for the post was dug in my garden and the fence erected. One evening I came home to see the neighbour had taken down the shared fence without consulting me."

Did you consult the neighbour before erecting the fence in the first place? Theres every chance you could have come to some sort of arrangment with them to replace and split the cost if they where going to do work themselves anyway or planned around it with their fence replacements. I think after that point it would have seemed like consulting wasn't required if you didn't. (edited)

Who according to the deeds owns the fence that put a new fence in front of? Seems pretty strange to erect another fence in front of an existing fence?

You really need to have a word with the house owner as these sorts of things can escalate quickly.

If after having a word you get nowhere and the new fence is clearly on your land, I’d personally send him/her a letter giving them x days to remove it else it will be removed by you and the costs to do so will be passed onto them

It sounds like you have now lost part of your land, all be it a matter of less than foot, the boundary has now been moved. This could cause problems if/when you sell.

Who actually owns that side of the fence? We have neighbours on each side of us and we only own the right fence, the left belongs to the neighbour on that side.

If they have damaged the fence by attaching something to it, it could count as criminal damage. I know you aren’t allowed to pile dirt against someone else’s fence as it will damage it.

I would look at the party wall act.

I'm no lawyer, but as far as I know a neighbour has absolutely no right to attach anything to a fence on your property that belongs to you, or to paint it or to interfere with it in any way, and you would be well in your rights to detach his from yours, although would be wise to avoid damaging anything you remove and need to return it.

As far as taking the old fence down, that depends on who it belonged to, and more importantly whose property it was on. (edited)

does your property deed state who is responsible for the maintenance of that fence? did you discuss the replacement of the shared fence before installing your own?

Think your house insurance legal cover will cover you for this kinda dispute.
If you don’t have a good response from your neighbour, maybe contact them and see if they would help.

I too am in the same situation - worse is that the neighbour is not very pleasant.
All the houses in the street have the same style fence where one of the properties has the posts and then both parties attach a fence panel either side. Sadly the previous owners took down their fence panels and when the new owners moved in they took it upon themselves to paint the back side of my fence panel. I did point out that it wasn't their fence panel. I guess I wouldn't have noticed only for the fact that it was brown and they decided to paint the rear of our fence panel green with some of the green dripping through. Add to the fact that they decided to screw in solar lights to the top of our fence and you see it's not about being petty.
Sadly as others have mentioned you need to involve solicitors at this point, that costs money and can impact the resell value of your property as you need to declare any disputes.
What really annoys me is the fact that we had no issues with the previous owners who frequently offered to assist with gardening and odd jobs (they where a retired couple and had plenty of time to kill!)
Ah if only Ramsey street was real!