Is it Norton Endgame?

I can't think of any bearings that small in a Norton powerplant. 1/16" is smaller than I've come across even on a bicycle. Is the engine standard or might someone have put an axial thrust race on the cam or something ? 

Mmm. May be a BB from behind a grease nipple?  

I bet its been in the oil tank for years, quite possibly from when it was made. Is the 'ball' polished like a ball bearing? if it isn't and looks like a lumpy ball of slag it could be from when the tank was welded together. Then, when the bike fell over in the field, it got its chance to stick to the magnetic drain plug.
   How hard is the 'ball'?  
Has the oil tank been repaired in the past? The bottom mounting is notorious for cracking around it. If so, the repair could have introduced the 'ball'   Think about it, for the 'ball' to exit the engine it has to get past the scavenge pump suction strainer (the one in the bottom of the engine) through the oil pump and then through the oil filter and hence to the oil tank. 

Thanks for your post Alec,

It just confirms I'm not the only one who experiences these 'one thing after another' and 'normally, that shouldn't happen' things that drive you mad with both anger or frustration. They do make for added experience and good stories afterwards. But I do feel your frustration, especially in that field.

First Mick, I would have never thought of those grease nipples. Can't think where these could in any way could get into the engine, but they indeed contain tiny balls.

And Peter had the same thought I had: welding spats. Try 'sticking' one to your vice and hit it with a hammer. If it's a welding spat ball, it should flatten a bit. Alternatively, you can try to file (you'll feel immediately), but holding the ball will be a problem.

After your stroke of bad luck, it's easy to think in the bleakest scenarios, but Peter's advice does sound sensible. Sometimes things aren't as bad as they seem.

Crumbs Alec - sorry to hear about your (mis) adventures!  How annoying and it does test your faith in the bike.  I hope you can get to the bottom of things - I'm sure the collective wisdom of the forum will be able to help.

Picked my Commando up from Pugh's in Ledbury and enjoyed the countryside on the ride home.  Not a great place to stall at the lights though...

Keep us posted on progress.

Tim - you are right - too much beer and not enough thought!  Yes tiny BB's but nowhere near the internals of an engine.

I do not recognise this point- CAM COVER? HEAT RESISTANCE ON IGNITION COILS?

 Fortnight later found the problem  to be, no heat resistance on the Sparx  electric  ignition coils under  the cam cover..
Obviously it wasn't the problem as apparently the bike has failed again. A misfire then cutting out and a later restart, can be due to a flat battery-no charge-easy to sort. But in this a professional mechanic is involved so money should have changed hands under a professional situation and you still have the problem-so back to mechanic to get the job right.

Need more info on what the bike has been fitted with. What ignition? Original wiring? A new voltage regulator or still on the original zener? What  carbs  has it got fitted?  etc etc 
 I;m with Al on the 'no heat resistance on ignition coils under the cam cover'   Is that your statement? or from your mechanic? Because it sounds like nonsense. 
For your 'mechanic' to say the ball on the drain plug magnet probably came from his work bench just highlights the fact that he is sloppy in his work (allowing the drain plug to roll about in the shite on the bench and not cleaning the drain plug before fitting it)
What work actually has this 'mechanic' done to your bike? 

... with comments re the mechanic. I would steer well clear in the future.