David Mackay has left a comment on the earlier thread, saying that in fact he maintains that Gordon Hughes' estimates of windfarm deterioration are incorrect.
Christopher Booker did not check his facts: Booker asserts that "David MacKay ... could not dispute [Hughes's] findings", but this is poppycock. You can find a technical report I wrote, pointing out a significant flaw in Hughes's analysis here or here. Another paper is about to come out in a peer-reviewed journal, by Iain Staffell and Richard Green, which does the analysis properly, combining wind data with weather data. There is a decline in wind farm output, but it is much smaller than Hughes asserted.
As Guido would say: "Developing".