Gotcha. Now, I have called in the big guns in the trib household (i.e. dh, ds is a little young to have started his Telecoms training!) and we both think that the problem may be that you are not an ntl user, i.e. it's chucking you out because you're not on ntl equipment.
Another possibility is the firewall, although you said you'd turned that off, and I wouldn't expect it to behave like that.
I'm going to test this theory when one of my non-NTL friends is online by getting them to try and log into my account. With my previous ISP, anyone could access the ftp site as long as they had the username and password, but I wonder if ntl is different.
To rule out the possibility of it being an ftp problem on your machine, you could try ftping to, say, the Tiscali account you mentioned before using the DOS prompt to see what it says. Or indeed just via Frontpage. Do you know what your own ftp settings are, i.e. for your own ISP? Again you can try that too.
Equally if any other MNers are reading this who don't use NTL broadband (or dial-up) and want to have a go, they can try and ftp to NTL by:
Going Start->Run and then type cmd
This opens a DOS window
type ftp upload.ntlworld.com
If you get what I described, i.e.:
Connected to upload.ntlworld.com.
220-
220-##################################################################
220-
220- Welcome to upload.ntlworld.com
220-
220-##################################################################
220-
220 upload.ntlworld.com FTP server ready
User (upload.ntlworld.com: (none)):
That proves my theory wrong. If you do get it, just hit Enter and it will say you need to put in a username and close the connection. Then type quit and you're done, you can close the window using the x at the top or type exit.