I have no idea what happened but after more fiddling about with some of the suggestions above, my Defer updates function is now restored!
Windows 10: Defer Feature Updates Blanked out After Recent Update? 
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New 20 Mar 2017 #11
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New 20 Mar 2017 #12
It's 'Defer', not 'postpone indefinitely'. Once the Anniversary Update became 'Current Branch for Business' it could no longer be deferred. The Defer option will now apply to the Creators Update (due shortly), but only until that too becomes CBB.Steve C said:
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