Thanks for the input guys: next time I'll use a Pratt-Shelby knot. Conclusion: the silk for most DuChamp ties is too thick for a full Windsor (let alone a quintuple Windsor).
I'm about to get the jacket vented. Just for interest here's the "non-window-paned" shades I was offered today (W. Bill, Sackville Street, London):
I am told vents don't need an exact match
Edited by Hacking jacket - 3/25/13 at 8:32am