so it turns out that digi cameras do develop interference at higher isos which has a similar effect to film grain. given my fetish for natural light i've had my camera permanently set to 400 to enable shooting without a flash whenever possible. i just set it back to 50 as an experiment and my pics instantly look so much better.
it's a pity the speed settings are squirreled away in a menu somewhere, as i'm not great at checking my settings at the best of times. my manual slr has a big label on the bottom saying "check film speed!" after one too many lost films from not doing so.
i'm trying not to think how much better sooo many of my pictures would have been had i realised this a bit sooner :roll: