Im not a lawyer and I doubt you will find one on here willing to offer what youre asking. However, it has always been possible for a leaseholder of more than 2 years standing to extend a lease on statutory terms of 99 years with a peppercorn ground rent. In your shoes I wouldnt worry about it and would just sell as is. Its above the 80 year threshold for Marriage Value, (which I understand may be abolished anyway) and you can always offer to begin the process of lease renewal yourself and pass that on to the new buyer. It wont cost you anything and it avoids the 2 year wait for the buyer. You should get full market value or close to it if you sell now.