Owain, The good news is that it's a runner again, but as always another obscure facet of the design appears, to keep you on your toes. On your plunger design, the original primary case seal behind the clutch is little more than a compressed fabric ring, and is a prime source of leakage. The later S/A set up has the felt ring (and sliding plate). The original type seal swaps easily with a modern seal of the same size...shaft, housing, width.
On the plunger design this seal can only be changed with the gearbox removed from the engine unit, and as you know this ain't a 5 minute job.
The breather exits the engine just under the chainguard extension on the back of the primary case. It may have an extension pipe, if still original, leading down and clamped adjacent to the sump plate. The gearbox oilseal on the sprocket is another common source of leak, also the mainshaft/drive sleeve bushes.
So its time to get out the big mirror and the rags, clean off as much muck as you can and observe where fresh oil appears. In the short term, if it turns out to be that pesky seal, try reducing the amount of oil in the primary case. My trick is to drain the case, start the engine, the add oil until a reasonable amount of oil can be seen flicking past the filler hole, rather than a cascade. This shows the chain is just dipping in the oil, rather than deeply immersed. This level helps keep oil off the clutch.
Plenty on the Forum recently about the seal, and how folks have solved the problems of holding it in place. Worth checking gearbox oil, in case that is overfilled and working its way out.
Looks as if you have a concentric carb fitted, if I have followed your posts correctly.
Bergs...I can't make up my mind about EP90 and phosphor bronze bushes. Anyone know for sure if this is true or an urban myth? Glitter is more likely to be particles worn from gear teeth in use rather than stuff eroded by some chemical reaction between bronze and oil additives, to my simple mind.
Swarfy.