Thanks for the replies. A bit more research and I've discovered essentially what I'm dealing with is Bellows Extension Compensation on a large scale. I've jumped from 4x5" where I never needed to worry about it, to UULF 1mx1m upwards, where it is essential.
Now I'm standing inside my camera, trying to work out the exposure with my light meter, when possibly what I should have been doing is working out the focal length or magnification, and apply these figures to the Belows Extension Compensation calculation.
Ideally, I'd still like to try and somehow meter the exposure from inside, but again, as I am taking the reading from the film plane, the light will not pass through any aperture, and thus that figure should be irrelevant. So when setting/reading the meter, that's what I proposed f1.0, because its the largest the meter will go.
Hope I havent confused anyone! Love to see or hear of some examples of workflow doing shots 1m x 1m (40"x40") or larger.