Well, it looks like I have a quirky USB stick. Just to be sure, I took a clean version of TC (6.1b1) and tried another USB drive. fstab shows
/dev/sda1 /mnt/sda1 ntfs-3g noauto,users,exec 0 0 # Added by TC
and this mounts RW without any additional parameters and works fine.
When I umount this drive and plug in the original 'problem' one, it mounts and works RW as expected. However, if I re-boot and plug in problem drive, fstab shows
/dev/sdb1 /mnt/sdb1 ntfs noauto,users,exec,ro,umask=000 0 0 # Added by TC
dmesg shows this
[ 553.361927] sd 4:0:0:0: [sdb] Test WP failed, assume Write Enabled
and I can't RW.
So ntfs-3g is working fine, and I seem to have a quirky thumb drive.
Thanks for your time Béla.