Slightly embarassed (!) to report the measured WL length was 1256 - the hull unfortunately turned out a little heavier than planned plus I used an old fin and bulb combo from Roger S, resulting in a slightly deeper draft. The SA was 0.994388m2 - rating 9.99. Despite that the boat absolutely ghosted along when there didn't appear to be any wind at all.
I know the tumblehome was a bit excessive but it did mean that even when heeled the deck edge was rarely immersed. It's sad to hear you, and clearly others as well, haven't pursued the 10R class as I do think they are the most handsome of boats, even with 100mm chopped off the top! The wind at Frensham is often light and a few of us there are still looking at designing / building more 10s.