Leasehold - time left v saleability etc

[Loosehead]

I'm trying to write and article on the pros and cons of Leasehold flats etc. Can anyone point me at a source of information, what I can't seem to find out is:
a) At what point will lender not lend in terms of un expired years, therefor effecting the price. How is market value effected by outstanding lease years?
b) Assuming a lender demands for example a new 99 year lease how much is it likely to cost, for example on this site someone said that to restore a lease from 70 years to go to 99 years to go it cost �15K. Is there a formula that works this out.
c) Say a flat has only 10 years to run is it effectively worthless to the current leaseholder?
d) Is the Freeholder forced to allow renewal of the lease at any time?
Seems to be a grey area and all the googling I've done seems to turn up loads of things but not what I sctually want to know. Thanks

[Catso]

I don't know the answer to all your questions but I do know that when the time remaining on a leasehold gets down to 80 years, it starts to affect the price. IE the value of the leasehold starts (gradually at first) to fall, and the price to purchase the freehold starts to rise. Or so I've been told by a solicitor, in respect of me selling a freehold to a leaseholder.

[themas]

Go to www.lease-advice.org and download their very helpful booklets.
In most cases, it is possible for a leaseholder to get a 90 year extension to the present lease term, so if it is now 70 years it would become 160 years. There is a booklet on valuation. This is very complex and normally done by specially experienced surveyors.
With the sort of exception the previous post mentions, I've heard that main stream lenders don't like lending on leases with less than about 60 years to run. It probably depends on the lender's policy - you could try asking the CML if there are any guidelines.