Financing a lease extension

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Has anyone had experience of a financial institution being prepared to fund a lease extension? I have a flat with 16 years on the lease with no current mortgage but, if I am to pay the premium to get the lease extended under the LRHUDA, I will need to borrow on security of the flat. So far I have not found a bank or other lender who will lend in order to fund the extension despite the fact that, on drawdown, there will be a 100+ year lease for them to take security over. They all seem to want there to be a 70 year lease in place at the time of application, which seems to me to be irrelevant as they will, on completion, have an adequate term. Any thoughts?