Thought I would share this response to my enquiry to Camden:
Thank you for your enquiry regarding extending the lease.
We do not issue voluntary lease extensions nor give an indication as to price. You will have a statutory right to acquire a lease extension once you have held the lease for two years. You would need to appoint a solicitor to submit a formal notice under section 42 of the Leasehold Reform Housing and Urban Development Act 1993. I can advise that a new lease would be for the remaining term of the existing lease plus 90 years and only upon receipt of this notice would we instruct our valuer to calculate how much the lease renewal would cost, and respond with our own counter notice.
Further information on the lease renewal process can be obtained from an independent body called the Leasehold Advisory Service (LEASE) http://www.lease-advice.org
The LEASE website contains lists of recommended professionals. We recommend that you appoint a solicitor and surveyor to act on your behalf as you will be liable for the landlords costs from the moment you serve a valid notice on the freeholder.
The initial notice should be served on the following address:
Housing and Adult Social Care
London WC1H 9JE