Anthony - apologies for such a slow reply
Most splits of this mature happen when part (A or B) is sold off so that part is removed from the original title by the buyer and they get a new title all of their own. They submit the TP1 plus form AP1 and fee. 
You can ask us to split it as follows but we can refuse
 

It is possible to split or subdivide the registered title of your property, however, if it is not as a result of a sale/transfer, it would be at the Registrar's discretion, and you would have to disclose your reasons for the request.

If you wish to lodge an application:

  • You must apply on form AP1, notifying us that a transfer is not required.

  • You must explain in a covering letter the reason why sub-division is required. 

  • You will need to clearly identify the extent on a plan that complies with our registration requirements. You can find out information about plans in HM Land Registry plans: guide overview (PG40).

  • If the land is subject to a mortgage, consent to the subdivision of title must be obtained from the lender.

  • There is a fee of £40.00. Cheques/postal orders should be made payable to HM Land Registry. We do not currently accept card payments for applications.

Please be aware that we will only consider your request once a formal application is made, and we may refuse to proceed with the application if we consider that dividing the title would not be desirable.

If you choose to prepare the application yourself, send it to the following address:

HM Land Registry Citizen Centre
PO Box 74
Gloucester
GL14 9BB

Under our advisory policy we can give customers factual information and procedural advice about how the land registration system works and how to make an application, but we cannot provide legal advice. In view of the complexities of the law in relation to this matter you may wish to seek legal advice.