When doing client side NAT'ing on Checkpoint boxes and using manual NAT creations the process differs between platforms.
IPSO is probably the most straight forward:
SPLAT I found I had to do the following;
cd $FWDIR/conf ; vi local.arp
cpstop && cpstart(Thanks @Levente Szabo for the update - don't have to restart the box)
Only now would the device respond to arp requests for that IP address - i.e. when creating manual NAT.