FZS600 has stopped working

[Stretchah]

Just wondering if your bike still has the original plug caps?
They were renowned for developing micro-cracks that let the spark track to earth. It’s sometimes possible to see that if you try starting the bike in the dark.
Note that a plug sparking OK when it’s out is not always a good guide because the compressed fuel/air mixture in the cylinder is harder for the spark to jump though.
NGK SD05F are the replacement caps most people use.

I've ordered a few bits today including new plug caps, better safe than sorry...thinking it will be a process of elimination to be sure