I've soldered the cable following this exact drawing: http://www.denfordata.com/bb/download/file.php?id=1263 - the same that you've highlighted, Peter. What do you mean by 3 wires?
A serial link will work with just 3 wires (but not all links) they are TX,RX & common: pins would be 2,3,7 in a D25 setup .
Now you need to say what com port is on your pc as there are 25 pin and 9 pin (also there can be a parallel port that has 25 pins) So are you sure you are not plugging into the PP?

Have you done a continuity check between the pins on the plugs to see if you have any shorts etc.

I don't know your Denstep card a pic. of the system might help.