As a coda, we took the touring tandem out today and made it round the whole loop. 69 miles according to the bike computer (67 according to the GPS). 5 hours, 44 minutes of riding, 7 hours 54 minutes elapsed.
Since we were last up there, the footpath cut-through to the Waddesdon estate had overgrown rather a lot, so we cut down to the A41 and followed it through the village - so no close-up view of Waddesdon, but as you leave the village you do get a good view forward to Quainton and its windmill and back to the roof of the house. We also did a spot of cyclo-cross (mostly as smooth as the tarmac) to get a good close-up of Claydon House.
The weather was close and muggy, but oddly cold - more like March than May.
http://en.wikiloc.com/wikiloc/view.do?id=2855875