Hello Simon,
Just found your renovation posting having just acquired a 33 GT, with yes you've guessed it leaky windows. Though this is fairly common with older boats as you say. I am very interested in how you go about replacing the window seals in situ as you have done and also what seal rubbers you purchased from seals direct.
Liked the tip on the decorating tape as well, I will have to give that a try this winter. Was it the Prodec Low Tac masking tape you used?
Thanks for such an extensive posting, its much appreciated.