Yes Don, Winky did indeed provide a good answer, & my response was somewhat flippant aimed more at the poke at the tories, which is completely inaccurate; the new subsidy contract for offshore wind is £57.50/MWh – a significant fall from the previous average of £117.14/MWh. As a comparison to £92.50/MWh (controversially) promised to the French & Chinese for Hinkley Point C.
I have been enquiring on the www & have answered my own question - or at least the purely technical question I had in mind: The National Grid have to balance supply & demand to deliver power at 50Hz, this requires constant regulation & very short reaction times. Forecasting the demand is a constant moving variable – outside expected demands they have daily weather changes, days of the week, even post big TV viewing brew ups to add to the mix. Each different generator type has a different start up time, reliability & cost. The NG use the constant (practically) unchanging up to 8.9 GW from Nuclear & its 48 hour startup (& shutdown) time together with the more flexible up to 34GW capacity of CCGT to provide the majority of UK power. Wind is used as a significant support to the NG balance with these other generator types; the problem with wind is it's the least predictable & can never be at 100% of the installed capacity due to unpredictable local variations in wind speed, maintenance & others, at this time the NG plan for using 40% to 60% of the total UK installed capacity. (and that 40-60% plan really answers my question) This is under constant review as more capacity comes on line over the next years. However the regional power generators do show some exceptions; Scotland produced 125% of the regions total demand from wind for some days over last christmas, I expect we will see more of this over the next years.