knome's tip seems to achieve what you want without ill effects. When looking at the permissions on the NTFS partition they, strangely enough, look precisely as if the umask had been set to 000 in /etc/fstab (read, write and execute for everyone -- not what you'd get with umask=022, which I thought was the default), but fstab remains free from umask options, and the error messages are gone.
I don't know whether fstab is still being overwritten (according to the msec log it was last modified at 7 o'clock this morning and that may have been before I rebooted), but at least nothing in it has changed: no umask option has been added.
So, thanks, knome, you may have saved tvmonster's day.
Thanks also to melodie, even if I didn't have time to test your ideas.