has my upstream dslam got stuck on 66999 or is TalkTalk doing some sneaky bandwidth limiting?
It's not Talktalk. It's the OpenReach DLM which has banded (capped) your line.
This is usually due to too many resyncs in a short period of time.
Only a great deal of patience, or a DLM reset from a visiting OpenReach engineer can remove the banding. Unfortunately sometimes banding gets stuck, and the DLM doesn't remove it as it should.
Unless you can convince Talktalk to send out an OpenReach engineer then you'll just need to wait and hope it removes itself. Talktalk definitely won't do this unless it takes your line below the minimum guaranteed line speed. Even then it can be a struggle to get Talktalk to do anything.
No amount of resyncing the line will change anything. It may even make DLM think the line is unstable and lower the banding.