Hi Rory,
I don't necessarily think there is anything wrong with the stock filter, but am wondering how well it actually filters in a dusty environment. Compared to other oil bath filters I've worked on it seems kind of small, and the wire mesh seems kind of coarse to do a thorough job. The original Listers had a tall, oiled, felt air filter which seems more like the dry-type filters. For most purposes, when the engine is in an engine house, the stock filter is likely completely adequate, but mine will just be under a tin roof, exposed to lots of saw dust and I am wondering how well the stock one will filter under those conditions.
Ray