OK, it looks like I'm fixed - thanks to a midnight phone call to a young fellow in Support. It's so good being able to chat to "local" Canadian support.
We unplugged everything (router and modem) from the hydro for 15 minutes. Then, we powered on the ST516 modem and did a hard reset. i.e held the recessed button in for 10 seconds plus, until red lights flashed. I kept the wireless router out of the loop, by plugging directly into the modem via the ethernet port.
We did a series of tests (logging in to test@test via the modem), but everything came up super fast with great line stats. So, everything got plugged back together, and it's worked great (pings back down to 25ms, speed back to 6mb dl) night and day since then.
I don't think I can tell what actually fixed the issue - because with this problem, the very fact of troubleshooting the issue in this way doesnt really make clear whether simply disconnecting my session and reconnecting (i.e 2 clicks of the mouse) would have done the same, and maybe I just got lucky with where I landed.
The young tech suggested the 15min unplugging was relevant, as was the hard reset of the modem. I don't know enough to doubt him. All I know is since then, i'm good.
Anyhow, am sat back streaming the start of the Ryder Cup with a vat of coffee, a bottle of Bailey's, and my ankle strapped up from a rather harsh game of rugby the other night
Hope you can get the same result.... (DSL, not ankle!)
p.s I forgot the young lads name... but given the amount of flak TS support is getting right now, can I just say he was informative, helpful, and was a pleasure to deal with.
I love it when support folks tell me what they expect to happen BEFORE I tell them what has happened. It instills great confidence.