You're referring to "marriage value", that is the increase in what a property becomes worth following a lease extention, as opposed to what it's worth now.
To the lay-person like me, it is complicated, but to the people who's job it is to sort it all out, it really isn't. Added to which, whilst a longer lease will make this property more sale-able (mortgage companies get twitchy when there's less than 80 years left), I doubt it will add much to the value of it (given the very high service charge and the council tax banding, it's going to take a lot to make it worth more), so the marriage value point may not come into it.
However, you only have to google "leasehold reform act" to see there are a great many changes going through right now, some of which will take a few years, and all of it for the better, so I can well understand why a buyer might want to steer clear of anything to do with lease extensions unless the seller has already sorted it, as the buyer will get a better deal on the lease if / when the changes become official. Right now is not the time to be extending a lease if you're the party who doesn't have to.
Given the choice, there is no way I'd ever buy a property where the lease had not been extended as I wouldn't want the aggravation. I have bought two leasehold flats in the last 15 years and I didn't look at anything where an extention had not already been given.
Whilst I don't know Leatherhead at all, on the surface this flat doesn't look to be the best investment going, and I am so sorry for the OP that she feels she's backed into a corner.
However, while the photographs look like the place is very clean, it could do with a tidy up, and that door needs to go back on the washing machine as it screams of somewhere that hasn't been maintained.
I'd also like to know who the target market is, because unless it's people with children, there's no way I'd be mentioning in the sales particulars that it has child-friendly gardens and a playground - a lot of people buy flats to escape small people.