Thanks Shortbus, I've noted your comments and have designed my machine to perform as you suggest.
In fact I could probably raise my lower speed setting if required, since 20hz is unusually slow and may never or only ever rarely required because it is very slow but may be good for fiddly work.
I'll know more once I get to get the feel for it later...
I've seen where guys add a small computer type fan to their motors when expecting to run very slow for extended periods.

 

Good news, motor run-on has been minimised by programming and no need for additional resistor.

So unless something goes bad on me, I am done with this thread.
Thanks for watching and commenting.
A special thankyou to the folks who had offered help with suggestions that has led me to this point.
I hope it has either amused or educated you all along the way...

Russ.