Well it seems my old ADSL Nation 1.0 finally died, but in the weirdest way.
My line was 100% stable back to 3dB 73.99Mbit but I was tidying up some cables so unplugged the modem and plugged it back in, and couldn't get sync.
Tried a different PSU, tried my Plusnet Hub One, nothing. Removed the filter and used the one that came with the Hub One, and synced.
How can the filter fail in such a way that it has zero impact, until you try to resync?