Obviously not all property will be sold with a 99 year lease � my understanding is that this only begins to become a problem once you are selling with less than 20 years � where the purchaser might be looking at the cost of re-newing the lease, or at least taking a hit on the sale price due to the lease shortness, at the time they come to resell. I don�t believe that it is any big deal to re-new a lease, other than money.
Although I have never owned a flat, a far more important consideration is the annual fee, payable to the company/individual owning the freehold for the land. I know many people living in flats who resent paying extortionate fees to management companies for very little.
I would find out how much this is, if you don�t know already � it is rather like council tax, you have no control over the annual increases.