Incorrect entry

We own a property registered with the Land Registry as Fayleys Lodge, Aldworth, Berkshire RG8 9RL. This property is, in fact, whilst 1 building capable of being split into 2 separate living spaces. Several years ago we decided to separate them for living purposes. This lead to each being separately rated for Council Tax purposes and the Valuation Office valuing each for Council Tax purposes. Someone, we assume either the local Council (West Berkshire) or the Valuation Office appears to have notified the Land Registry and now you have 2 entries - one for Fayleys Lodge and one for "Flat, Fayleys Lodge", for which you hold no title information - for the reason that there is none as "Flat, Fayleys Lodge" is part of the title covered by Fayleys Lodge! We have just realised this as we wish to change the address for Council tax and postal services from "Flat, Fayleys Lodge" to "Gardeners Cottage, Fayleys Lodge". To do that we need to show proof of ownership but of course as "Flat, Fayleys Lodge" is a part of the title covered by Fayleys Lodge, there is no title! There is no reason for the Land Registry to have an entry for "Flat, Fayleys Lodge" for the reasons stated above. What we think is the correct outcome is that the entry for Fayleys Lodge is amended to say "Fayleys Lodge and Gardeners Flat". In that way we can have a title deed that we can use to update the Council tax and postal service. 
Is this possible and how do we do it? 

Henry - a couple of things to note here re the address information you refer to. The first, and one you are already aware of, is that it is the local authority that manage and create postal addresses. If there's a change needed or a new one created then they are the ones to do that.
The second is that in this case the 'Flat' address was created by Ordnance Survey and presumably after some new/changed local development 
My advice would be to check and confirm the postal addresses with the local authority. And once you have that confirmation you can use our Contact form to submit those details and our support team can look at updating the address information we have/links to registered info as appropriate
Public guidance: dealing with land and property - GOV.UK