Sounds like you may have fallen foul of the following if your leaseholder became aware of "marriage value" since originally offering you the lease at the lower price, particularly if your lease has been extended recently :
"Because buying the freehold significantly increases your home's value, the landlord has the right to take extra money from you over the market value of the lease extension. This so-called 'marriage value' gives the landlord half the net increase in the value of your flat."
The marriage value can vary from a few quid to hundreds of thousands depending on the increase in value of the flats as a result of you owning your own lease. However, the shorter the lease the greater the marriage value, but I believe that if the lease is longer than 80 years the marriage value is (or effectively is) zero as a result of a reform of the Leasehold act in 2002.
I'm no expert I picked this all up after a googe search
This government department is probably the one you want as they deal with these issues.
http://www.rpts.gov.uk/
Dave