Neighbours wall falling down and he just covered it with a fence

WTF that's bad form imo. Fair enough the wall is actually theres but it also acts as a boundary between you two and as a courtesy this should not have been left like that, especially saying nothing and you wake up to this shambles, it's very negligent on their behalf. I'd at least knock in and invite them into your back garden to fully assess the mess and ask them to clarify have they intentions to remedy this. As it stands (pardon the pun) this is very dangerous and poses a great risk, especially when windy stormy nights come in.

It could be argued that block was removed on the OP's side. I cant see how a block could be dislodged but the timber post behind it is still intact. If that was knocked from the neighbours side then the post would have snapped.

OP can’t touch the wall as it doesn’t belong to him and isn’t on his property.

Wall can’t be removed anyway probably because the fencing is anchored to it.

OP don’t approach the builder as he doesn’t work for you.

Talk to your neighbour and simply ask him what he intends to do.

Even with the overhang, is it still within your neighbours property?

If so.. curious from a legal perspective why he would have to fix it if he doesn't want to.

It looks like it poses a serious danger.

I'd presume that within the law you're entitled to some type of safety within your own property, but how this effectively works when the danger is a neighbouring property is unknown to me, but I did find this article which has a section called "boundary buildings and repair"

As the wall is clearly leaning into your property and can only fall one way, I'd be thinking that you need to formalise the communication to the neighbour by stating the concern in a letter. I'd provide a timeframe to have the safety issue resolved and if they don't respond within that time, contact a solicitor to communicate it in a legalese way and proceed with the above. I'd be stating that you request a replacement wall in line with the existing building regulations, and you could state that you wish the replacement wall to be on the party line (therefore giving him back 4 inches of unusable garden 😉 ).

If as per the information provided that if the neighbour wants to maintain the wall fully on their property and not a party wall, then yes, the neighbour should pay for it fully.

(added missing "if")

Perhaps he's just going with the cheap/lazy option for now. Looks like he didn't even bother removing the old fence panels before putting the new ones up.

You say he didn't present an option of going halves on the wall, but did you suggest it to him? Maybe he can't afford the full price of a wall, but wouldn't have a problem if the cost was split.

Isn't it entirely possible that the neighbour isn't fully aware of the state of the wall? I think it's possible he's not even aware that the new fence panels are just bodged in on top of the old ones. All this talk of confrontation, demands and ultimatums (ultimata?) seems a little extreme until the OP has established that the neighbour knows what is going on behind the new panels but has decided to be an arse about it.

Good neighbours are priceless. I'm not suggesting we should tolerate bad behaviour from neighbours for the sake of an easy life but definitely give him a chance to fix things. Approach it as a conversation. "We need to decide what we're going to do about this wall. Do you want to drop in for a look and we'll see if we can come up with some ideas? Obviously we can't leave it as it is."

Not necessarily shared responsibility. I've seen estates where it was written folio that right side rear fence was in in ownership of the property. I think that is usually done where fences with a back and a face are used so that each garden gets one good side and one back side to look at.

If the previous owner of the neighbouring property built this wall, it should be assumed it was built just their side of the boundary so the op would own right up to the face of the wall but not the structure itself.

The main responsibility of the neighbour here is the safety aspect. They cannot allow unsafe structure to fall into neighbour.

Been looking again at the pics, that new fence cost a few bob and an odd thing to do if one was intending rebuilding the wall! It looks fairly permanent and not as if they are going to just remove the right side to repair/replace/demolish wall. The original fence looks like it was attached to that wall and is obviously disintegrating. Wonder what is supporting the new fence? I can't see anything obvious like new post holders or similar. It's surely not also somehow attached to the old fence/wall?

There is quite a gap when you take into account the wall/old fence/new fence, maybe neighbour doesn't care though, sure what's a foot of garden! It's hard to see solution here, it's obvious neighbour should have repaired wall when his digger damaged it and then taken off old fence and put the new fence panels there. It's like they were just sick of the mess of wall and fence and just didn't want to see any of it anymore and covered it up with the panels instead of dealing with it correctly.

If you take down the wall now as it's falling anyway or it falls itself plus the fence neighbour should at least pay for disposing of the stuff although again I wonder what their new fence is supported by? Will you then gain the foot of garden up to his new fence! Are they likely to be happy with that scenario!