How can I extend my lease if the other freeholder won’t communicate?

Hello everyone, I am looking for advice/help to resolve this.

I purchased a leasehold flat with 50% freehold interest in 2002. It is a detached property with three flats. The ground floor is owned by me and the first floor is owned by a lady who also has the other 50% of the freehold.

I was a first time buyer and didn’t give too much attention to the lease. During peak COVID (2021), my mortgage was coming for renewal and I approached the mortgage adviser, who informed me that I only have 57 years of lease left. The other freeholder has 999 years lease. The flat doesn’t have service charges or ground rent. The only ground rent is paid by 2nd floor leaseholder which is £100 per year.

I have been trying to get my lease extended, but the first floor lady is NOT communicative and is completely ignoring all my requests.

What can I do? I need help as I am struggling to secure a re-mortgage deal with banks due to the 54 year remaining lease.

Thanks,

Rajesh