The neighbours may have an easement on your deeds/registration mentioning the necessity to enter and repair.
With a 1919 property, I'll bet there is nothing mentioned though.
In which case, they will certainly have a right to do so that has been established over a long period. In the absence of any written indication, then simple common courtesy says that they must tell you they need to enter, and arrange a suitable time. In my experience, this would be the legal position as well.
I doubt very much if any right to enter without your knowledge has been established.
With the Water Company.......... they may well have a statutory right to more or less come and go as they please, especially where there are Environmental Health concerns.