The switch should sit within ,and close to, its allocated space within the raised section of the handlebar grip.
Hopefully it's just a case of lining everything up, a bit of trial and error. Have a go at loosening off the brake lever and repositioning the switch ,or twist the handlebar grip round - you should hear a healthy "click" when the button is pushed/released & it should be quick to return (sort of spring loaded).
You've done yourself proud mate, and I'm glad we've been able to help
Martin's wiring kit is every bit as good as you say, but it's aways nice to hear "independent" appreciative comments, so thanks for your posting
Plus it's always good to see another c5 alive and back on the road !