You have 2 possible causes of action here. The first one is in common law nuisance quite simply because this is a nuisance.
The second cause of action is interest bus although trespass is not usually actionable unless some kind of damage is caused.
If the neighbours need this access for anything required to preserve their property (maintain it) they are entitled to have reasonable access under the Access to Neighbouring Land Act but from what you have said, it is just more convenient and not essential.
If the neighbours take no notice of you, consider getting a solicitor to write to them threatening them with an application to court for an injunction and substantive legal costs if they do not stop allowing their workmen to go over your roof. At the same time, tell them that they are responsible any damage caused to the roof and you intend to have it inspected to confirm that no damage has been caused.
If they ignore that letter then you have no alternative but to apply to court for the injunction and legal costs.
If you have house insurance, you could well have legal expenses cover that might pay for the solicitors costs and the court application.
Can I clarify anything for you?