I don't think there's enough metal to just drill a new hole - as you can just about see in the background, the hub shell is higher the other side of the flange, so I can't actually get very much nearer the center than the existing hole, and it's all cut away between the spoke holes. Though it is interesting that's bencooper's suggested solution, as he's the one person on here who I think actually has one of these hubs, so I'll have another look. Really not at all keen to go with one less spoke, as I don't want to risk the strength of a wheel which has a lot of load, on which I'm not all that stable, but can easily do 15mph+ on. I'll have a think about bolting to the bolts you can see in the picture, but I'd really rather not touch them as I'd be messing with the delicate internals.
If not, make a slightly larger stainless steel flange, and use the existing hub holes to bolt it in. Then you need new spokes. But it's all doable at home with a bit of careful bodging, and much cheaper than a new hub.
I think that's the winner at the moment. I'm wondering whether it might be possible to just take out the spoke to the left of the broken one and bolt a bit of plate to that hole, as the spoke forces ought to mostly balance each other, meaning very little static load on the existing hole, and the highest loading being due simply to decrease in spoke tension as it's ridden. Probably a better solution to take out 3 spokes though to spread the load (if I only take out 2, the force won't be balanced on the plate).
the older the gear, the stronger it usually is. They didn't have fancy cad, dynamic modelling in them olden days. Everything was overbuilt.
Not actually that old at all - less than 10 years I think, just rapidly superseded. Certainly far from overbuilt, quite the opposite - that's the whole problem.
Oh, and thanks for all the help.