Hello and thank you for your question. I will be very pleased to assist you. I'm a practicing lawyer in England with over 10 years experience.
may I clarify with you please are you seeking to if possible to place the rail on the side of the fence facing your neighbour?
yes this is set inside the cocrete posts
thank you. what he proposes quite lawful subject as follows. There is no law that provides that you must have one side or the other facing your neighbour. There is a social convention that you face the "good side" of your fence towards your neighbour but this is not legally binding and you can place posts or cant rails or indeed other objects on your neighbour's side of the fence.
there is one caveat to the above which is why I say above " subject as follows".
no objects are fixed to your neighbour's side of the fence they overhang your neighbour's land-in other words, all such objects including posts rails trellising and so on must be within your boundary. if such items overhang your neighbour's property, this can amount to a trespass and your neighbour can require you to remove such objects from his land or that overhang his land
is there anything above I can clarify for you any further?
no other questions i feel the reply is you have provided covers the issue raised .
thank you. If I can assist any further on the matter, please do not hesitate to revert to me