Well it's recovering very slowly - and without the help of Plusnet, who after a week have only just decided to respond to my support ticket and are now claiming it's a copper joint fault.
There has been a resync at ~6am every day, without fail.
Day 1 saw interleaving raise to 2000/275 and sync drop to 33/13
Day 2 no change
Day 3 saw US raise to 16mbps whilst US interleaving went up to 360. DS interleaving dropped to 600.
Day 4 saw DS interleaving drop to 600, US interleaving removed completely.
Day 5 has seen US back to 20mbps and DS interleaving off.
As it stands, I'm left with interleaving off again and a sync of 33/20.
Full monty stats over the period
here, for anyone that is interested.