I filled the gearbox until oil came out (car was levelled using jacks). It seemed to take over 1.6 litres, although I did spill some. It should take 1.4 for a drain and refill.
I'll just have to keep an eye out for escaping oil. It's something extra to keep an eye on, along with the clutch master cylinder, inlet manifold, VIS motors etc, all of which are vying for my attention at the moment