Actually Nige, I did use the paper neg method to discover the light leaks. And after seeing where they were located on the paper negative, they were actually small hairline cracks in the darkslide.
These were easily located by placing the darkslide on a lit light table in the dark room, and covering the remainder of the light table with black cloth (in the dark of course). After letting my eyes adjust to the darkness, I turned on the light table and immediatly found the culprates.
To make sure the slides wre completely fixed, I decided to completely cover the inside of the slides with a black-out film, which I had used on the windows in my darkroom. A sort of VERY stickly opaque black film, I cut it to coer the entire inside of the slide, but not covering the edges that run in the tracks. This solved the problem and now I can bring the filmholder out into the light without fear of fogging.