FWIW - it appears that getting anying above a stable 3Mhz signal is a bit iffy at most.
Have spent best part of 2 weeks on this (hour or 2 at a time) but have now ordered an external clock generator (Adafruit).
Anyway, with minor mods, if anyone wants a 3Mhz signal, with minor mods (just to clean it up), the above code will do it (BTW it is a sketch not a scotch.....)
Stan