If you own a leasehold flat you don't actually own it, well only until the lease runs out.
Most mortgage lenders like over 70 years which will affect the value if only cash buyers can buy it.
The 1993 Leasehold Reform Act says flat-owners are legally entitled to get 90 years added to their lease at a fair market price.
You need to have owned the flat for at least two years.
For short leases the price can go up a lot.
Normally the cost of extending will easily pay for itself when you come to sell.
Click here for a lease calculator. (Scroll half way down the web page.)