Took the GF for a spin this afternoon.
Good news on two fronts: clutch doesn't slip now - using st 40 oil in primary plus an extra two clutch plates for more friction. And the N/S fork leg no longer leaks - at 3rd attempt. Second attempt found the oil seal wasn't being properly clamped by the SS seal holder because either the seal is too thin or the rebate in the holder was too deep. Thought I'd cured it by machining some spacers to sit atop the seals, but it still leaked (though much less). Remedy was to fit rubber seals instead of the metal-clad ones I'd been supplied, and adding the later funnel-shaped distance piece instead of the circlip.