After enormous amounts of faffing (~4 hours, I believe), I've booked directly through Eurostar a 1-4 person appartment in Ste Catherine. It looks like you get a reasonable amount of privacy between the two rooms, which is good. But argh, that took some doing.
We're going to Brussels on Fri 24-Aug-07, 10:43 train, arriving at 14:02 and returning on Sun 26-Aug-07, 20:28 train, arriving at 21:56. This means we have the bank holiday Monday to recover from the trip.