Thanks again to everyone for their expertise.
I had - as mentioned earlier - tried asking Eurostar, and got nothing useful; when I pressed them they told me that there would be no London-Amsterdam through trains on the dates of the engineering work ... which wasn't what I'd asked them, of course. (Despite that, if you try to book such a London-Amsterdam train on their website, it still doesn't own up to the known, and admitted by them, lack of service that day, but says to try again later.)
Following that non-answer from their customer services e-mail address, I did go back to them one more time. I asked them point-blank whether - irrespective of being able to fix actual timings yet - there would or would not be trains at least as far as Brussels on the affected dates, and, specifically, on 2 November. I've had a reply from them this week saying: "We will be running trains to Brussels on these days but yet to release the schedule until we know what impact it will have on the timetable. We expect these to be released over the next few weeks."
I have to say I'm minded to believe people on this forum in preference to believing what Eurostar tell me!