The Premium

THE PREMIUM is the sum of firstly the landlord's loss resulting from the grant of a new lease with an extra 90 years at no ground rent and secondly fifty percent of the marriage value. Marriage Value is the increase in value attributable to the "marrying" of the freehold and leasehold interests.

While a lease is still over 80 years in length no marriage value is payable but once a lease falls below the 80 year threshold (even by a day), marriage value can result in considerable extra cost.

If your lease is still more than 80 years you should extend it before the 80 year threshold is reached.
It is even more important to act quickly when the lease has fallen under 80 years because the Premium increases as the lease decreases i.e. it will be more expensive next year to buy the lease extension than it is now.