Well, it appears my suspicions were correct. I do seem to have quite a low cadence and it doesn't seem to matter if I am doing a tempo 5k or a long slow run. 158 steps per minute seems to be the golden number! I am going to try C25K+ Stamina again tomorrow and see if I can stick to 160 for a decent length of time.
Is there anything I can do to up the cadence a bit? Other than just "move my legs faster" which just knackers me out very quickly.