The Herald 1200 rocker box has a large oil filler cap which contains oil socked wire gauze. It vents directly into the air through holes on its underside. It works in conjunction with the crankcase breather on the side of the engine. The service notes say that you wash out the cap with petrol, leave to dry and then re-fill with engine oil. If I remember every 6,000 miles. Basically you need a bigger washable filter on the end of the pipe coming out of the alloy rocker box.
Dave