Either I'm going (more) mad or somewhere in the 40K RPG range I saw a secton with additional information on several of the Navigator houses, above the names of prominent ones mentioned in the core book. Would anyone know which book I might have seen this information in, or even have a website with information on the houses available?

The idea is that the player creates his own House as part of his character's background.

For inspiration to create your own House, check the Lexicanum entry about Navigators: http://wh40k.lexicanum.com

Darth Fanboy said:

I think you're thinking of the section in the Inquisitor's Handbook for Dark Heresy. There's not much information there, though: it's more a list of 2-3 houses...

There's a bit of information (but not a lot, really) about House Nostromo in the Haarlock's Legacy series for Dark Heresy.

There's a really neat little section on a renegade Navigator Cult operating in the Jericho Reach in the Achillus Assault book for Deatrhwatch.

Apart from that, Navigators are (as of yet) not particularly well served for background information.