Just to close this off - I managed to fix my problem by taking the IAC apart and giving it a thorough clean.
First thing to note, there is no bi-metallic spring of any kind inside the IAC valve. I can only assume this was an old design that has now been done away with. The only thing controlling the valve is the electromagnetic black coil (with the 3 pin plug)
To get the IAC apart, I had to drill a hole in the end of it...
This then allows you to tap the valve out with a small nail and hammer. I used a stubby self tapping screw to seal the hole back up.
All I did is clean it up and press it all back together again.
Some more pics - sorry about the quality.