Problem resolved today. Even though the Openreach psu was powering up the modem, it was obviously borked by the power surge. A new Netgear router with integrated modem works fine. The only thing I hadn't changed out, because it appeared to be working fine, was actually the culprit. I'm not an electrical engineer but my guess is that there's some form of smoothing circuit in there that was affected and messing up the signal.

Good job it's sorted as my provider, CONS, via Timico, still haven't been able to confirm whether the issue has been escalated to Openreach. I will mail them tomorrow but I doubt they'll bother to read it. Next job is to change supplier.

Cheers