Technically there is, it's supposed to be 8 feet. But people let them grow bigger. The main thing is to keep on good terms with the neighbour and try to agree a trimming regime, maybe sharing the cost? Good luck. I think they should need planning permission to buy them in the first place!!

they do need planning permission in the first place but it depends how long thats been inforced . i would do as volunteer sais and if know joy go to the council but its not nice falling out with nieghbers so be carefull.