(Reply to EMnut)
Non-faffage is important, especially at this time of year. In my experience winter 200s tend to be slower because (a) it's cold - I seem to travel more slowly when it's cold than on a warm summer's day, and (b) some of the ride will be in the dark (some might get round before sunset, but your post suggests you won't be one of them) and again riding tends to get slower in the dark. So to give yourself the best chance you will need to keep down the time spent in cafes.
Bantam Tearooms in Chipping Campden is very nice, but can be very slow. And once you're sat down in a nice warm room it's a bit more difficult to get going again. I would suggest getting something nutritious and easily digestible from the general store opposite the "town hall". Again, Summer Tearoom in Malmesbury can be very slow. The garden centre before you get to Malmesbury might be quicker; I've never used it. When I've done PS I've always stopped at the garage just outside the town. It's pretty soulless, and thus doesn't encourage you to linger!
If you can get yourself in a small group after Malmesbury you'll probably finish more quickly. It's reasonably flat, and in a group you can share the effort and the navigation in the dark.
You'll enjoy it - PS is a lovely ride. I've done it 10 times, in a wide range of weather conditions (but usually wind!)