Ok, so it seems like I've figured out the problem (thanks greatly to you folks!). Useful when you know what to look for!

I just went to remove the washer and cotter pin to find that they are completely missing, so I can just pull the head of the wire off the assembly with my fingers. This likely explains the high idle when the engine is at operating temperature too I expect?

Full disclosure: I only bought this car a couple of weeks ago - pleading not guilty to screwing this up!

Are the missing pieces difficult to come by new? Seems I need to order a few bits...

Thanks again!