updating freehold to include a room over an unregistered alleyway that is a right of way

I have lived in my house for 22 years and am now selling. I have a bathroom that is located on the first floor over an alleyway between my neighbour's house. The buyer's solicitor has identified this as a flying freehold which needs to be added to the land register. How can I update the land register to include this information and how long will it take? This needs to be expedited because of the house sale and I am in a chain

Polly - you need to make an application to register it or, if you think a mistake was made on registration report the error and have us investigate.
Much depends I suspect on whether your neighbour's title refers to the flying freehold but yours does not and whether both properties are registered. 
If no error has been made on registration then the conveyancing and deeds for both never highlighted it. If that's the case then the option is to claim ownership through adverse possession or the neighbour transfers the bathroom to you.
I'd suggest discussing with your conveyancer as to what's registered and how/why and consider the best way forward to resolve it to enable the sale to go through 

Hi there, my neigbour's title does refer to his half of the flying freehold and alley way (subject to right of way). My neighbour's title does not refer to my bit of the flying freehold or the alley way underneath it. I think it is an error that my part of the freehold and alleyway are not reflected in the register. 
I have reported the error but will follow up with further information. 
Thank you 

Polly - it looks like the respective rights of way over the alleyway are referred to as appropriate but not the room(s) above 
I suspect, but won;t know, that the room(s) above were not flagged/highlighted in the original deeds submitted when the titles were first registered. 
As they were registered back in the days of paper the support team will need to look back to see what paperwork we still have on record. They will then respond to advise what options exist to resolve matters now

Thank you. I have the original paperwork crowncopyright 1962. 

Polly - it will be the original deeds/documents that count here. Namely the deeds/documents relied upon at the time the title(s) were first registered. 

okay i have all the documents going back to when it was built. Thanks for your help