The first operational use of Decca was June 5th 1944 to sweep the corridors to the Normandy Beaches. There was a lovely story that two of the Decca Engineers 'kept' a decometer at home to see when it came live and then subsequently claimed that they were the first civilians to know that the invasion was On.
On the 330J offshore Sumburgh, it appeared that the FO spent 95% of his time on the flight keeping the thing going, I seem to recollect two chain changes and three chart changes (though it was usually two chart changes if any trainer apart from Eddy Forde wasn't about!!)