Thanks both for the elaborations / contributions :-)
Just to see, cleaned / oiled my old washer plates and spacer, reassembled the inner and outter baskets onto the gearbox shaft and torqued the clutch centre nut to 40 ft lbs (what my haynes manual says for a 2k4 dt175).
Can now detect slightest of movement between inner and outter baskets, and inner basket spins quite easily with a single finger.
I don’t know what I have done wrong or right, but I think this seems to be as it should be!
Good news I hope.
Thanks.
Mike

I usually get the ball out by putting a dab of grease on the end of the mushroom shaft.  Ball sticks to grease and out it comes.

Talking balls--umm--if it wasn't there i doubt you could adjust things for the clutch to separate but could be wrong. It is a 3/16 ball just looking at partzilla so i guess the adjuster on the pressure plate could take care of that if missing but i'd expect adjuster to be close to dissapearing in the locknut. Even so, the clutch would still separate & work how clutches do-- but the pushrod ends wouldn't like not having a ball to revolve on. Well one half revolves & actuator end i guess doesn't revolve when being 'actuated' but would revolve- 'all of it' when clutch released. Confused?-i am lol.

I agree RT325...  and on some models Yamaha experiment with using rounded end rods and no ball.  The modern parts lists don't show this.

As you said, if it was possible to thread the pressure plate adjuster in far enough to "kind of" work, I doubt you could get the pointer aligned with the nub?  And if you don't get that lever geometry correct, the clutch won't disengage correctly even though it will work somewhat.

Ballls… dab of grease… nothing. Magnet tool arriving tomorrow. 
I’ve read two rods can cause a lot of hot and can weld together if the ball is missing. Urban myth? I don’t know but fortunately the end of my pressure plate rod still looks fine.
 

Well… one question categorically answered:-)
photos.app.goo.gl/JbebqpRmc2P1JE6F8

Good work... hopefully the initial "stiffness" you reported when turning the inner hub that now was reported gone was the issue? 

Just make sure clutch is adjusted properly and fingers crossed!

Wow, that's a cool little magnet stick. I want one.
Mind you i dropped a gudgeon pin into the crank of a honda rancher quad the other day & luckily my big magnet stick hooked it out. Was sweating there for a bit lol.

Laser 7026 magnetic pick up tool. 4mm diameter so fitted nicely down the bore of the shaft.

Inner basket now rotates nicely but problem was nothing to do with the ball. Took the baskets off,
inspected, oiled, reassembled and now rotates ok. Don’t know what I have apparently fixed.
Not made time to reassemble and adjust the clutch and see if clutch action and gear change is now ok.
Fingers crossed.
Mike
 

When assembled & before cover goes on just try moving it along in gear with clutch pulled in--just to be sure to be sure it frees up good. Of course with hub turning free anything else can only be plate related if you can see the pressure plate moving out. Expect a little drag if the plates are pre-oiled but if dry to my mind i think it should be totally free to wheel.