Most of the routes up and down the ridgeway between Stokenchurch and Princes Ris seem ok, but most of the routes through the woods on top of the ridge near Stokenchurch are very muddy. The ridgeway path is more variable, but mainly better north of the M40.
The frost in the mornings this weekend fixed things until it melted.
I like the area around Warburg Nature reserve in winter, as there are a number of long tracks that tend to offer good going, and some are quite rooty and fun. No GPX,sorry, but the area is between Bix Bottom, Stonor, Russels Water, Crookley Green.