Am 23.06.2013 00:13, schrieb Paul Corfield:

Thanks all for your suggestions.

buy a normal Z16 One Day Travelcard
ticket on the Friday morning and then before you travel to the airport
go to a National Rail, NOT an Underground, ticket office and ask for
an extension single (one way) ticket from Boundary Zone 6 to Gatwick
Airport. This should cost £4.90 single.

OK, I will do it this way. Less fuss and not too expensive.
--
Kai Borgolte, Bonn