It goes down behind the inner primary cover in a fairly gentle curve and is clamped to the gearbox mounting plate with a 'P' clip...It exits onto the road just below the plate...Ian
I have had a couple of WD M.20s where the crankcase breather pipe had a small hole drilled in it roughly in line with the gearbox sprocket and assumed this was top deliver oil mist to the rear chain and no I have never had any issues of the oil mist on the rear tyre.
Thanks folks, I figured BSA would've got that right.
Back in the day I owned a 650 twin Yamaha. Its engine breathers exhausted directly in front of the rear tyre.