So last night found me sitting down at a table, with the guide on my laptop screen, disassembling the lens. Pretty quickly I spotted that the diaphragm spring was not attached to its arm (again) so I knew what was needed to be fixed. I also knew by reading the guide that I would have to pretty much strip the lens down completely to get to the spring. In the process I also discovered that there was something (oil or grease) on the aperture blades so while I had the lens apart I took a couple of cotton swabs (q-tips) and carefully cleaned the blades; making sure that everything functioned correctly and smoothly.
Now, easy part over, comes the re-assembly and insuring the lens works as it should. It took me a couple of attempts to correctly line things up so the lens focus movement moved correctly but everything went back together and it seemed to test out okay. I attached t to my camera and tested the aperture and focussing - both of which seems to be working correctly.
I'l be interested to see how long it will keep working before either the spring comes off or the blades gum up but at least now I know I can save myself some money and just fix it myself.
Sunday 16 September - update
Guess I didn't clean the blades that well as they are sticking again. At least the spring is still attached! I'll just have to do a more thorough job of cleaning the blades and see just how long the lens will keep working.