Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
1. Your Solicitor is quite correct- the Leasehold interest can't be removed until completion, as this would be extinguishing your existing legal interest in the outbuilding/land.
2. The document transferring the Freehold to you will be done by way of a Transfer of part (called a TP1, and will say it is a transfer of part of the existing Freehold title to the whole site) and the Land Registry will therefore know it is forming part of the Freehold title relating to the whole site. They will therefore create a new title number for you, without any issue.
3. When your Solicitor registers the TP1, he will also submit a Form CN1, which is the application form requesting that the leasehold title be extinguished.
I hope this sets your mind at rest.
Thanks for that. On Point 2 I don't understand. Currently Freehold title T1 has 2 lease titles L1(flat) and L2(outbuilding). The TP1 will result in in a new Title T2. How will land registry know that L1 stays with T1 and L2 is associated to the new T2?
The TP1 will include a Plan of the land to be sold to you, and the Land Registry always look into the existing title numbers to see which parts affect the land you are purchasing.
You really don't have anything to worry about.