CGT on lease extension

[venture53]

I owned a house as my main residence which was converted into three flats land put into my BTL business. The three flats were held under a single title initially. I decided to remortgage the flats and had to put each flat under a separate leasehold title and create a single freehold for the 3 leaseholds.

The leasehold is solely owned by me. The freehold is held jointly with my husband. I own 99% of the freehold and my husband owns 1%. No money changed hands when freehold/leasehold structure was created.

I am looking to remortgage again and the mortgage company wants a longer lease on the flats as the current lease is about 80 years only.

My question is if the freeholder ( I and my husband) decide to charge a premium for extending the lease, will there be a CGT and if yes, how is this calculated?

Thanks for any advice extended.