The tree owner is within his rights. You are allowed to cut off what overhangs your property but there's no right of access to his garden to do it. The law says that whatever you cut off should by rights be returned to the tree's owner, but I know how it feels when a neighbour does this without mentioning it first and it doesn't make for good relations. But if you have permission to cut off what overhangs your garden you may do it. There isn't anything you can do about the height of the tree except enjoy the flowers, which must be magnificent!

Check your title deeds. It is almost certain that there will be a clause in them that says something like reasonable access to your property is allowed for maintenance to your property by your neighbour. If that's the case then it's safe to assume that you will also have the same rights.

For example, if you needed to go onto your neibours land to repair a wall to your property they have to allow you.

So, if this is the case and the tree surgeon said that the only way to prune the tree is via your neighbors land then by law they cannot deny you access onto their land. I would consult your deeds, find out if there's a clause in it to this effect and if there is one point it out to your neigbours.

To keep good relations, when you have done all this, it
might be a good idea to consult an intermediate party
to speak on your behalf. After all, everything grows, and its not your fault.

I think I read somewhere that you also have the right to ask for a tree or hedge to be pruned if its blocking the light but good luck with that one, think that ruling was brought into play when the enormous leylandi hedging began to appear..

@Myron you are correct about having a right of access into a neighbours garden for repairs, but pruning a tree is not a repair and could be seen as trespass or harassment if they go into the garden without consent.

Thanks for your advise folks. My neighbours have tried being as fair as possible, offering to pay for all the work which is deemed necessary. The tree has been causing problems for years. It is quite massive, the sunlight is totally blocked from their garden for a huge part of the day and then in autumn the leaves block drains and gutters as well as making the drive and paths lethal if they become wet. But, no matter what my neighbours do, the owners of the tree flatly refuse to either prune it or allow access for anyone else to do it.
It looks like stale mate and to cap it all, the local council don't want to know, it being on private land and a private dispute. :(

In other words your poor neighbours are up the creek unless they are prepared to pay out a fortune in legal fees, my daughter works for a solicitor, always says neighbour disputes can take years to solve and the only one who wins is the solicitors..

I would play them at their own game... Wait until they go out, whip in their garden and prune the offending tree to your boundary then tell them you did it from your land after all... Simples. Even if they find out you went onto their land, there's nothing in the world they can do long as you have only cut it back to your fence, as you are entitled to do, and not damaged any of their property. Trespass is a civil offence and they could only sue you for restorative damages. As you didn't damage anything, they wouldn't win.