Flying freehold not included in title

I have owned a property for 18 years. I erroneously believed that the passageway to the side of my terraced property (also used by another 4 houses to access their gardens) formed part of the registered land for my property. This passageway and a small strip of land between my house and garden are unregistered (where the neighbours walk to get to their gardens). Above the passageway is my bathroom and third bedroom. Only my property extends over the passageway and I have maintained this area since purchase. The property that sits on the other side of the passageway is not able to access this to gain access to their garden. The Land Registry plan of the property does not include the rooms that extend over the passageway and no reference is made to a flying freehold in the title. What application would be required to rectify this with the Land Registry and what evidence is required? Is is possible to have the passageway registered to my title or would it be limited to a flying freehold?

Kathryn - anything is possible and I would strongly recommend seeking legal advice/assistance to clarify how the law views your ownership of the property inc flying freehold and then the unregistered/claimed land
For me your starting point would be checking to see what, if any, old deeds/documents you have that might shed some light on the flying freehold specifically as that would be your starting point/focus. 
I suspect you will have nothing as whilst the property was first registered back in 1974 (see A Property Register) you bought in 2007 and doubtless relied on the registered information and the seller handed nothing 'extra' over at the time.
The reason I mention this first is that if you can show that the original deeds included the flying freehold, and an error was made, then that would be the basis of amending the title as it stands. In the absence of such evidence though you are likely to have to 'claim' ownership of the flying freehold based on your 18+ years of ownership/occupation
The second aspect is then the unregistered land beneath the flying freehold and to the rear of the property. That is likely to be a separate legal issue as others have access to it and use that access for their own properties. Such actions will very likely negate any legal claim you have to owning that land. And as a result claiming a right of way over the land might be all you can consider although 18+ years is too short a timescale and you may have to wait until 20 years have passed to make such an application
Our guidance on claiming ownership and/or rights of way/access are contained in our PGs 5 and 52 but it is the legal complexities you need to unravel before trying to tackle the registration ones
Land registration: Practice guides - GOV.UK

Thank you.  This matter has been raised as I am selling the property.  I think dealing with the flying freehold will be the way I proceed in the circumstances, hopefully on an expedited basis. 

Kathryn - noted and appreciated. Have a read of this thread as well re possible timescales re such applications - it just gives you a rough idea of what might be required re process even with expedition. Will help manage expectations for you and a buyer as appropriate - Adverse Possession - Claimed Land - Impact on Timescales · HM Land Registry 
Do discuss the access/right of way aspect with your legal rep as well as the buyer may ask and may ask you to provide a statement of truth/stat dec re your use of the access over the time you have been there