Answer to your question, Wired: thank you for your advice. It was set to 1000 (1Gbps). (First place I looked when bandwidth dropped)
I don't believe it is at my Router end (though I can't guarantee it's integrity further without diagnostic tools). I would hazard a guess it is more fundamental i.e. configuration not set correctly at the "Distribution Hub or in the Head-end - Management systems?"
My Router has been performing robustly, circa 95+%, without intervention for a couple of years (minimal resets)...but then suddenly dropped in Bandwidth (ave 88Mb from an ave110Mb ) directly coinciding with the upgrade from 100Mb to 500Mb. Incidently, the upstream bandwidth has increased as per expected