Dave Robinson is a very nice chap and knows Ducatis very well as he has been working on them for about 30 years! He also offers very good value for money. But make sure you discuss with him and list everything that needs doing on the bike as I am not certain that he checks rockers / cams / valve clearances as standard for instance. Also if you want special racing oil, etc, you might have to supply it yourself.
Other reputable independents in the area are Patrick Foley (I think that he is around Surbiton / New Morden) and GR Ducati in Wimbledon.