Hi Hywel,
Windsor is 'unique' in that it has a much later start time so it can be hot (26c last year) which really can take its toll. But as it is being run later this year and my window is telling me heat shouldn't be a factor but might explain some people's lack of PBs.
The course is as they put it undulating but looking at the stats about the same as Wycombe.
It does get a bit congested but less so when you are a 1:30 runner just make sure you line up acordingly (they have banners).
Anyway good luck next weekend hope you break the 1:30!
Colin