[sideshowb]

update: the £50/MWh and £90/MWh are both commercial bids that will have factored in probability of generation to the best of the bidders ability. the companies making them will fail if they go hugely wrong. (though in the nuclear case taxpayer might be left with cleanup)

You are correct that the non-constant nature of wind means you need to cost in some storage, or transport via the grid (generally it'll be windy someplace or other). But probability of generation is not the issue.