Hmm. I did both races in 2002 and 2003. In 2002, I did the same time for both. In 2003, London was 14 mins faster. Although the bike is flatter in Windsor, it is also 41.5k (though I'm not saying it took me the extra 14 mins to do this!)
Both runs have a heartbreakingly steep (but short) bit in them though. And you have to do both 3 times.
Windsor is the best race though.
BB.