The Cobra Mist radar instellation on Orfordness was designed as an Over the Horizon early warning system. The '4' ninute warning at that time was deemed much to short and all bases in the East of England plus the civilian population needed a greater time span. Cobra Mist was more for the early detection of Soviet aircraft heading West towards the UK and ICBM's were its secondary role. [Read all the web sites on this system].
Although the excuse for winding it up in the 70's was because it didn't work that well there is a conspricy theory that it got up the Russians noses so much that they started in some way to jam it. One con theory was that it was being jammed from out side the 12 mile limit by a Soviet ship or submarine.
There was also unsubstanciated reports that all these different beams floating about in the area especially from Cobra Mist that it was affecting local car ignition systems and local television reception.
My old buddie who was a Police radio surveillance expert suggested that there could have been extraneous RF/EMP from Cobra Mist causing havoc locally and it could not be filtered out so that is one reason for its demise. The other was the Russian jamming.
Obs