Neighbour has made a huge mess of my garden

[Streetwitch]

The landlord of the house next door to mine has got his gardener to chop down my ivy hedge. He has reached over the wall to do this and the whole lot has collapsed into my garden and completely flattened my main flowerbed.
It's made a huge mess and there is no way I can deal with it myself.
It also provided me with privacy so that's now gone as well.
I spoke to him last week and he said he would come around and deal with it but now he says it's not his problem because it was mainly growing on my side.
He's never said anything about being unhappy with it and if I'd known that he was I would have trimmed it, I fully accept that it was very high.
Does anyone have any ideas about how to get him to take the mess away? He has an assistant and the van so it wouldn't be difficult for him.

I think legally any cuttings of things growing onto someone else’s property technically have to be disposed of by the owner of the tree/bush etc

eg if a tree branch from my neighbours tree was hanging into my garden I could cut it down and put it back on their property.

Taken from gov website

When you can trim hedges or treesYou can trim branches or roots that cross into your property from a neighbour’s property or a public road.
You can only trim up to the property boundary. If you do more than this, your neighbour could take you to court for damaging their property.

As @boozeclues says, if the gardener has chopped the hedge on your side of the boundary, you can sue your neighbour for compensation for the damage caused. The fact it was mainly growing on your side is relevant only in as far as it makes it more likely that they did chop it on your side of the boundary. You may not want to sue your neighbour - that wouldn't help good relations. However, be clear that, as the hedge has been chopped on your side of the boundary without your authorisation (assuming that is the case), they are legally liable for any damage caused so they need to pay for removing the mess and making good the flowerbed.

No. You have to offer them the cuttings but they can refuse.

Ivy is not self supporting, Was the ivy growing up the wall ? Was it your wall/a shared wall or was the wall actually on his side of the boundary. If it's his wall then you shouldn't have let the ivy grow and he could charge you for repairing any damage to the wall. If it's shared then he would be allowed to cut it back to the top of the wall which could have caused the remainder to pull away from your side due to weight. It's not as straightforward as you think based on the info you've shared.

It was a huge mass of ivy sticking up into the air partially supported by a trellis, it was there when I bought the house and I think it was probably quite old.
I liked it because it provided a good screen.
He would definitely have had to reach over the wall to cut it but one good shove and I reckon that's what made the whole lot collapse into my garden.

It's a Victorian brick wall and no indication on the deeds of who owns it, so I reckon it might be shared.
It definitely did eventually collapse under its own weight but he must have had to reach over to cut it, and would have had to do a lot of pushing to help it in it's way!