Have you had a look on the official Scalextrix website? There are a few F1 tracks you can download(link) and there's a free track planner available. Perhaps these would be of any help.
What you also can do is, give in a F1 race circuit on the Internet, you'll have no problems finding information. Here's a Link to the Spa-Francorchamps track in Belgium.
I haven't seen a plan showing all the pieces used. If you had enough time you could probably figure it out with a track planner (UR3, TrackPower, etc), an image of the new Silverstone circuit, and pausing the video at every shot of the track to get some judgment of length and type of turns.
When searching for the video, though, I found this guy: http://www.youtube.com/user/mrstaff1984
He's done videos of several F1 tracks, and he's using Scalextric digital pace cars to show the action. Lots of great videos to look through, and I would bet he has plans of them all.
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