Simon
As noted above the PCV system is supposed to be a sealed system. However, a couple of other things you might consider checking. If you don't have a sheet metal baffle inside the valve cover under the
oil filler or PCV valve hole
oil can get thrown up by the rockers and exit the PCV and / or
oil breather / filler.
Also, the first engine I built (many years ago) I didn't make sure that the ring end gaps were offset from each other when I installed the pistons. I had a couple where because of the way I installed the rings the end gaps were almost perfectly lined up. This caused a lot of blow by and in the end I had to pull all the pistons and make sure the ring end gaps were offset correctly. Once offset and reinstalled the blow by was virtually eliminated.
Hope you find the problem...