Could be that DLM has kicked in due to the length of time the line was down.
Good point - I'll give it a few days and see what happens.
I've already tried asking Plusnet for a DLM reset, but they won't do anything at present as are prioritising other faults above speed issues due to Covid.
Can't see how I can of made anything worse - removing 4m of cabling, and jelly crimps, as well as getting the line away from a potential source of interference can only be a benefit (and the increase to upstream at least shows there's some potential imo)