I have now got the replacement spring and dismantled the door handle but with a problem.
The rod holding the spring mechanism is stuck solid, I guess with corrosion and I can't slide it out. I guess there was nothing much wrong with the original spring.
You can see the metal rod in the photos.
Apart from soaking it with WD40 and very lightly tapping the end does anyone have any further suggestions? I can't rotate it with grips and as it is just about all made from plastic I can't see heat is an option.
I will leave it overnight and see if the WD40 penetrates but I'm not holding out much hope as there is no free movement at all.
