I've googled the web and this group, looked on TfL's website and asked a
chap at a tube station, but haven't found an answer.

If you go to TfL's website and select Fares and Tickets from the menu you
are taken to a page that immediately mentions a leaflet
(http://www.tfl.gov.uk/tfl/pdfdocs/fa...ts-may2003.pdf) that you can
download, and that is where you can find the information. TfL's website is
a bit annoying like that.

Peter Smyth has already given the list though