Thank you. I am not sure ; still; which workspace to use. I am back using the default workspace having discovered the reason for one of the major problems (not able to zoom back in) and so now I can avoid the mistake; mostly; or recover from it.
I do appreciate all the help and expertise that is on the forum, and the help given thus far.
With regards to the dimension problem above, I have found a thread on here whilst searching for another problem, which explains why I couldn't dimension the larger radius. The solid curve was drawn as a series of facets, I turned this on (checked the box in properties) as was suggested. However I went into properties again and unchecked the use faceting box for rendering, and lo-and-behold I can dimension the radius at the end. why this did not work originally with faceting off I have no idea. However the dimension was a mirror image which threw me, but jiggling around with the workplane and UCS plan I got it the right way round.
Now the weird bit. I think this has something to do with workplanes, Whilst trying to work out why the tongue would not do a Boolean add to the rest of the base I managed to select the other circle and dimension it. This time the orientation was correct, however it is on the top face of the object along with the edge dimensions, and it really went bad. The holes disappeared and I couldn't figure what I did.
So I started again from scratch. This time I drew the solid, did an extrude to 20mm moved the workplane to the bottom, drew out the land either side of the tongue and did a boolean subtract. Yipee!!! That worked, except the dimensions were mirrored, Move the workplane and it fixed that.
TC seems to arbitrarily change layers without reason, this has caught me out several times.
I have another workplane which is being used to construct the next component, the body. Having checked in the design director as it is the last one created TC seems to default to it.
I think I arrived at your point in the latest iteration of the drawing. However I have degenerative faceting turned off, yet it still dimensioned the arc. I couldn't do this previously. I started again because I couldn't get the tongue to form a solid, so I formed it bw reducing the thickness eithr side of the tongue. That seems to work.
You guys out there have no idea how greatfull I am for the help. The answers are out there, but not knowing what terms to use in a search usually returns 'no matches found'. Oh and for your information, I am reverse engineering a drill grinding jig for an old English tool grinder. the tool grinders are plentyfull, but the jigs are as rare as hens teeth. The intention is to modify the unit so as to grind drill points and other tools.