Mike, by all means buy an inline check valve if you wish, Polly Palmer still sells them for around £40, but first I would re seat the ball valve that is behind the Oil Pump and fit a new ball 1/4" dia and spring part no. 68-353 and see how it goes. The spring does weaken as time goes on. I don't get any wet sumping on my A65 or A50 and this is after being laid up for three months or more.
Re the where do the oil in the crankcases come from, well the timed breather mostly but also, anywhere else it can find a way, via the main bearings into the timing side cover and the chaincase as it blows past the seal. It all depends on how much is in there.
Hope this helps
Chris