I don't know much apart from what Jen's already said. A few years ago we had to sell DH's parents' flat which had approx 65 years left on a 99 year lease as they needed to go into sheltered housing. We got the property valued by three different estate agents, all of whom very strongly advised getting the lease extended before putting the flat on the market, they also told us it could run to being extremely expensive as the freeholder can apparently charge what they like.
My inlaws were very, very lucky - the freeholder was an old lady who had gone into a nursing home and her son who was dealing with her affairs was so grateful that my inlaws had been good neighbours and looked out to his mum over the years that he set up a new lease for 999 years for no charge, they just had to pay his legal fees too.
Hope it won't be too costly or too difficult.