Ok, apologies for the late update on my behalf. I actually posted this up over a month ago but we lost some data when hackers took the site down. The long and short of it is that this boundary is correct after all !
The reason why is because the tide goes out a long way and the local police team patrol the beach and consider it part of the neighbourhood. Many thanks to the Home Office for getting back to us on this one. The topic will now be closed.