[pmahoney]

I have now solved this problem.
First I created a network object representing the internal network and static nat'd this to the single public IP address.
I then created an allow and forward rule for the public IP address on a specific port number and redirected this to the required internal IP address on the required port number.
I also created an allow rule for the internal IP address to allow traffic back out for the required port(s).

Note: I had to ensure that the internal machine had the firewall IP address as its default gateway or the connection failed.