Review added, and I've made the year 1977 accordingly. I suspect the release month of June stated in Music Master is correct (unfortunately there were no Decca ads at all in MM in June/July, though I am missing one issue - Decca's financial problems were probably beginning to bite), but it would be nice to get confirmation from a 1978 or later Decca catalogue if anyone has one; with any luck they will have numerical indexes with release dates in the back, like the 1974 and 1975 catalogues,
1984 Music Master says June 1977, which seems reasonable, but I might try and get at my 1977 issues of Melody Maker tomorrow to see whether it was advertised or reviewed; no luck in the Gramophone archives (via my online subscription) unfortunately.