Dirk GentlyRoyal Rother Of course you don't need the extra S. Mr Grammar-Pedant has disappeared up his own wotsit on that one.
Wrong, wrong, wrong!
Just adding an apostrophe after the "s" only applies to plural possessives - names are treated as singular and follow the "'s" rule, even of they already end in "s".
According to Lynne Truss, it's personal preference these days and not worth getting your knickers in a twist about.