Hi Simon -
Make certain that there is a PPTP rule in the Firewall section of the
web interface that allows clients - there should be a third tab next to
WAN (so there should be LAN, WAN, and PPTP VPN tabs). This may be
obvious (I hope it is!) but make sure you have actually established a
PPTP connection between the client and the m0n0wall device. Finally, if
you haven't, read the m0n0wall documentation on PPTP VPNs.