This is not really a question, but an answer in itself. I spent some time searching the forum, but was unable to find a solution, athough others - in threads that have, rather unhelpfully, been locked - have had similar issues (often with no stated resolution). I'm hoping that this helps someone with a similar problem.
The issue I had was that adjusting the FTP on my 1040 was not adjusting the FTP in Connect, either iOS or web (e.g., in Connect under Reports / Cycling / FTP).
I can see from other threads that sometimes this is caused by the "wrong" device being set as the primary training device, but my only other current device is a Forerunner 45S that does not record power or (obviously!) have an FTP setting. Others had suggested that it was necessary to complete an activity (why?) for the new FTP setting to be logged, but I'd finished and synched two.
Nothing having worked after repeated syncs, both using the app and via Express, I had another look at the whole thing today, and - eventually - discovered the culprit. I sometimes use the Tacx Training app on a tablet, and changing the FTP in that immediately caused the correct values to be shown in Connect. Clearly, it was overriding the value from my 1040, which is set as my primary training device.
I hope this helps others, but honestly - WHY? Sync should surely mean just that - surely logic says that the LAST update should be synched to all platforms, not the one Garmin thinks is the most important. And if it HAS to be the most important, then surely to goodness it's the primary training device, not a tablet which happens to have an app installed on it? This has taken me probably two or three hours to work out, two or three hours I could have much better used for something else.