Cheriton is right about LEASE, who are based near Old Street, and are a very useful source of info. You might also try CARL, for direct or indirect advice: www.carl.org.uk. I think LEASE have a lot of case histories which would give you a good idea of the kinds of prices and costs associated with extending the lease. If your lease is over 80 [or it might be 90, i can't remember the legislation offhand] years already then you may want to think about buying the Freehold -it might be much cheaper than you think and prof. costs similar. In my experience [limited though it is] you will only really be able to arrive at a sense of the right price by your own research, for which you'll need quite a bit, because professionals directly involved either don't know, have very general ideas, or are very blase and imprecise. Good luck.
Does anyone out there have any experience of buying a freehold for a block from Southwark Council?
I've heard that it depends on enough leaseholders within the block getting together - if anyone has been through this process, I'd be really grateful for anything they could pass on.
All I know is that you need at least 50% of the flats in the block to be leasehold. I have a feeling I saw quite detailed info about it on the ODPM website but not sure - have a google! Dont know of anyone with direct experience, but my block has now (I think) 11 leaseholders out of 25 and a neighbouring block has the required 50% and have thought about it - so I'll post if anything exciting happens!