After wasting time with everything from logs to packet-captures, I finally noticed a simple anomaly: one of my bluewin mail accounts had been switched to "offline" in Mac-Mail.
This particular "anonymised" account is used only for signing-up to web sites and doesn't get much traffic after the first "click here to confirm your mail address" nonsense.
I don't recall setting that account offline, but setting it online last night fixed the problem.
No other settings edited/changed.
All why/how questions remain unanswered -- but it works.
To answer my own question: "is it just me?". Yes
Chris