After 6 days of fun and games with the amp giving me loads of gyp I put things back as they were apart from the OPT primary being swapped over that is, all is well.
Turns out moving a few grounds and grid resistor ground terminations effect's the stability of the amp considerably.
So amp working like a charm again, stable with NFB applied. Gave it a quick test, and got 0.8% THD at 50w. The amp also now kicks out 162w before onset of clipping, though I don't leave it at that for long as the OPT is rated at 120w.
So I go from the sublime (0.8% THD at 50w for a valve amp) to the ridiculous ( 40v P-P 20khz oscillating hissy fits) all by moving a few grounds 6" or so. Video here -
https://youtu.be/S0Qhjzewr-4
A.