Fantastic! Thanks both. I just KNEW it would come down to something stupid I had done, or forgotten to do. In my defence I have only done this once before and it was some time ago, so it's not entirely unexpected that I would have completely forgetten that I needed NTFS drivers installed.

Seems to be working now. In Information -> Devices the correct free space is being picked up, and the disk mounted with no problems. I've even got it visible as a network drive on my Windows PC via Samba.

One minor suggestion: do you agree it would be nice if the absence of the NTFS drivers could be picked up by MountManager, rather than resulting in a crash?

Thanks again for your help.
Dan