After a lot of trial and error I’ve finally come to the conclusion that the NVidia ForceWare Network Manager can cause problems with the SonicWall VPN Client. I can’t say for certain as there are so many other things involved but after uninstalling it everything else seemed to start working as it should. I had the firewall disabled but it was still doing something to stop some VPN traffic going through. There’s some more detailed information on Experts Exchange