Just re-reading your original post again, I think you are looking at the wrong non-return valve (NRV). There are two inside the engine, one on the feed side and one on the return side. You have been looking at the return (scavenge) NRV which doesn't really do a great deal. It is the feed NRV which is there to help stop oil in the oil tank draining into the sump and a problem with it can result in wet sumping. On your engine, it is located in the inner timing cover and there is a grub screw that has to be removed to get at it. Therefore, it is a bit of a job to overhaul the feed NRV but it depends on how bad your wet sumping is.