This is a complicated business and legal advice will be essential. However I found the following formula in a Sunday paper. Hope it is of some help.
As a rough guide, a flat with a 100-year lease would sell for about 98% of its freehold value, but a property with a 60-year lease would sell for 75%. If you want a 40-year lease expect to get a 40% discount, while a 20-year lease costs about 45% of the freehold value.