The scoped waveform didn't show the main problem... at higher waveform frequencies there was ripple moving through the waveform (left to right)....I'm thinking it might be a 'beat' frequency relating to the difference between the pwm rate & interrupt rate (or it may be that my filter is just too woeful). Anyway, whilst pwm might be just the ticket for very low frequencies or static (or narrow range) of frequencies...I don't think it's the solution here.

ok, I can feel an R2R ladder coming on!