Pressure tested and it was leaking so a few squirts of soapy water and it looks like the drive side oil seal is just about to fail, Next pull the front sprocket off and check again.
It's 99% certain to be the drive-side oilseal.

The bubbles you can see are caused by air leaking past the drive seal and escaping out of the breather in the side case, soapy water easily shows the leak.
30mins later and leak tested with drive sprocket assembly removed, pressure tested with oil sitting in the bearing tracks the leak is obvious.

I took the engine out to change the gearing and service the engine but when I removed the Spark plug I wasn't happy with the deposits, I suspected the drive-side oilseal was on its way.

a couple of hours later.

The drive side oilseal is as hard as a rock it's got no flexibility, most likely caused by the petrol.