Hello I am a solicitor with over 15 years experience. I will try to help you with this.
That rather depends on the circumstances. A tree rooted on your property but overhanging your neighbours property belongs to you and you have a duty to make sure that your tree does not cause foreseeable harm to your neighbour's property.
How big a problem is this really?
Its a silver birch about 20ft high and the threatened action is based on trespass by foliage and seeds on neighbours property
Technically if your branches overhand your neighbour's property it is trespass. If it doesn't overhang but seeds/leaves escape into their land it would not technically be a trespass but could amount to nuisance or negligence.
It was my previous understanding that seeds and leaves constituted 'minor debris' and could not be reasonably regarded as nuisance is this no longer so?
However your neighbour would need to suffer harm before having an action that he could bring. You are on notice now that he thinks there is a problem and you might want to think about offering to cut back the tree if you think there really is a problem. Do these trees cause a blockage to your gutters or is your neighbour making a fuss about nothing?
There is no general rule that seeds and leaves can only ever constitute minor debris. That will ften be the case but will be matter of fact and degree and vary from case to case.
The gutter blocking when it rarely occurs is usually caused by a large oak on a public recreation ground. As it happens I rarely suffer from blocked gutters due to any leqaved
If leaves and seeds from you tree cause a blockage and consequential damage to his house then potentially you could be liable for the costs of repair. He has to suffer damage before he can take action. He also has a legal right to cut back your tree to the boundary. It might be that if you think there is any risk of a blockage you should take action to prevent it but if you are confident that there is no problem then leave things as they are if you want to.