Lease extension and mortgage

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Hi all

I have a question about lease extensions .

I have a flat with 65 yrs left on the lease and I'm looking to extend it by 90 yes. I have a quote from the freeholder that I'm happy with, it'll cost me £18,000. I'm trying to remortgage to raise the funds and I've told the broker.

Here's the problem.
1.The bank has surveyed the property but they want it in writing from my solicitors that i will be extending the lease. They won't tell me how much the flat is worth or how much they will lend me until then.
2. My solicitors have contacted the freeholders' solicitors who want £1800+VAT to begin the process, in advance
3. My solicitors won't give me the letter until then.

Now since i haven't got the valuation from the bank, (initially I was planning to release 25k equity), I don't want to get caught with a much lower valuation that will cost me a few thousand instead of getting it from the remortgage.

Is there anything I can do, something i can sign to say that "I will be using the funds from the remortgage to pay for the extension" and let the solicitors hold the excess funds till then?

Sorry it was a bit long, kinda at my wits end. Any help would be appreciated