Cannot access mapped drives over VPN

Hi, i would really appreciate some help on this problem.
I need to set up a VPN from Portugal to UK and access files on a WinXP Pro PC. I have setup the tunnel on Netgear DGFV338 router on the windows workgroup LAN and Netgear Prosafe client on remote WinXP laptop. In the UK the VPN connects OK and I can map the drives on the remote PC on the LAN.
However when the same laptop is used in Portugal the VPN still connects, but the drives are no longer accessible. I can ping the PC and I have set up a hosts file to resolve the names, but still no drives

Any ideas