ORAC,
The vast majority of civil radars are secondary only, hence the rotaing blades at windfarms are not a factor.
Are they? That's news to me!
NATS and CAA work closely on windfarms. I believe that the new radar at Trimingham is also desinged to mitigate degradation of NATS' en route radars as well as those of the RAF.
NATS and Windfarms - NATS
http://www.bwea.com/pdf/Wind-Turbine...n-Measures.pdf