I've recently used Ablitts in Sydenham ( http://www.ablitts-solicitors.co.uk/) for a lease extension (and sale) and was happy with their service. In the end it all went reasonably well in my case, but I know that it can be a very drawn out process, depending on whether you decide to do a "fair value" type survey first and come to an agreement quickly, or try to negotiate hard. For the valuation, I think I used a surveyor from Hindwoods in East Dulwich, who also did a good job.