That's pretty much what I'm planning on doing. The plan I'm following leads up to the SDW50, I'm also doing London so I'll be taking it relativley easy between the two races. Not 100% sure about the 3 weeks in between that and NDW50, lots of easy running and a couple of long ish runs but nothing fast.
I'm in for all four. I've only done the NDW50, that is quite a tough course with Box hill and Reigate hill. I didn't do as well as I'd have liked so I'm back for another crack. I held back too much and spent too long in the aid stations.