Right man, but I gave an incorrect cite, sorry.
What I had in mind was this http://www.gamedev.net/reference/art...rticle1698.asp by Abrash, not Carmack (Right company, wrong man).
Also the BSP FAQ from SGI is well worth the time, as are all volumes of the 'Graphics Gems' series by Andrew Glassner.
Sorry for the confusion.
Regards, Dan.