VPN over 3g Broadband Problem

Hi

I can access a VPN over my 3g broadband with no issues at all on my MacBook Air, however I just tried to do the same with a friends computer and who is using the same operating system and we get the following message in the ppp logs

"route_interface: write routing socket failed, File exists"

From google i see this can relate to subnet's but we are using different subnet's to access the remote lan.

Also the vpn connection does connect and authenticate over the 3g. It just won't route any requests for the remote lan down the vpn.

The vpn connection operates perfectly using a wireless lan or ethernet. It just appears to be over 3g it has this problem

Please help, as this is driving us mad!!!!