See this route around 49-54km. The ones I rode are very rideable. I did the bit on your route from Old Dean north and it is straightforward. I was on a road bike with slick 28s, so TCR style offroading. Other tracks around there looked fine, although some a bit muddy.
Ha, you look like you're doing a very similar ride to what I'd planned. I guess we've probably got similar coverage given we're west London TTers/audaxers. Thanks. I'll take the Mason with 28s and hope it's not too soggy.
How gravelly are the tracks in Swinley? Road bikeable?