Thanks for your reply.
Most Landlords are accomodating when it comes to Lease extensions. However, legally, as a Tenant who has lived there for over 2 years, the only Lease extension you are entitled to is an additional 90 years on top of the existing term at a peppercorn rent (ie zero rent).
If you are unable to afford such a Lease and the Landlord is not prepared to grant you a shorter term Lease, other than the additional 25 years, then I'm afraid you are somewhat stuck.
I do not personally know why he is objecting to you having a top up to 99 years, and there is certainly no reason legally why he should object or indeed why he is only offering an additional 25 years.
As you may be aware, most Mortgage Lenders insist that there is at least 60 years remaining on a Lease before agreeing to grant a Mortgage. Your 83 years would therefore be OK, but not ideal.
What is happening quite frequently nowadays is that whenever a client is selling a Flat/masionette with 60 or so years remaining on it, that we just agree with the Buyer to acquire a Lease extension on completion of the Sale. Therefore, the Buyer is happy and the Seller is also, as he does not have to find the monies himself, in that part of the Sale proceeds is used to acquire the Lease extension. Lease extensions do increase in price the shorter the term of Lease is remaining, but this is an option you could proceed with as and when you decide to sell.
I hope this helps and sets out the legal position.