Well, we did have some issues with NTR Pro struggling to connect 2 out of 3 times, (we had trialled the Ultimate version, not Pro) but they turned out to be due to the Physical quality of our ADSL line.
We moved last week to an LLU connection, so now we go through a new Line, and third party switches at the exchange.
End of packet fragmentation issues!
NTR pro now works well, nice and responsive, and the authentication issues have disappeared.
Our VPN should work too, will try it out when I get back from holiday.