Arghh bugger, I’m just re-measuring and it could be better if it was 19mm 
but 21 will still be fine actually, sorry to be confusing
Well, here it is at 19mm between centers. I basically redrew it with accurate dimensions and made sure it is a solid component so it will be printable.
Final.skp (74.7 KB)
I used more segments for the circles so they’d be smoother and I purged a whole lot of unused stuff which accounts for the smaller file size.
I sent ti off to i.materialise to check it and get a price.
Your’e a total legend Dave! Thank you so much for all your help 
Och! Think nothing of it. 
Is that a legend in your own mind? I thought you could see yourself as a mirror inspector.