SOLVED
I stumbled across this thread: https://www.reddit.com/r/unrealengine/comments/oqalo6/weird_light_squares_flickering/
Which had the exact same issue, and thankfully had an answer:
Raise the value of r.Shadow.Virtual.MaxPhysicalPages
I believe default is 1024, raising this to 2048 got rid of all but the rarest of artefacts, and I just set mine to 4098 for some headroom. No idea what other impacts this will have but it’s solved my problems!