I might be the last person in the world to work this out, but I’ve just found you can search for items in del.icio.us that match multiple tags in the del.icio.us interface.
All you need to do is insert a “+” between terms in the tag box at the top of the page (see screenshot below for an example).
This is, of course, documented in the help section on del.icio.us, but who reads help nowadays? :p
Then, suddenly, the “+” symbol next to tags listed in the “Related tags” made sense – clicking the “+” instead of the tag name adds it to the end of the current tag browse.
