Adding back into this thread with an update.
I received a visit from an engineer who, while replacing the Hub 4 with a Hub 5 (which, surprise surprise, hasn't done anything), informed me that "they can't do anything about latency". Which is pure drivel, honestly. Its very clearly oversubscription on the ISP, and my internet is unusable for gaming at the moment. Is there any way to escalate this issue?
I've also added in a new broadband monitor for the router, showing the exact same latency spikes through the day.
https://www.thinkbroadband.com/broadband/monitoring/quality/share/d4732c870cf0b8f2e48b9c1d0cd33f3fb0d2fe82