"Other tubes I've used (12BA6) that resistance could be 10meg it
behaves
well where with a 12EK6(12v space charge tube) was happiest
with 5meg."

I'm thinking of putting a 10 ohm pot in series with the filament of the
regen tube to see if making the filament less negative in relation to
the regen grid might work, maybe also using a bypass cap across the
pot. I don't have one right now but I'm going to play around with
different resistor values (like around 3 ohms) until I can order one.

Did you think about doing something like this when you couldn't get
your 12EK6's to work with a 10 meg resistor?