Our bike recently now requires two taps on the start button, a similar sound to a decompression lever not working, so the first push gets the piston partially over the stroke then the second one kicks it into life, has a new battery so not that, do they have an automatic decompression system for starting?

https://www.youtube.com/watch?v=vU0QPhhpJ-E similar to this?

[2015 Z Force 800 53/Trail EPS]

The 400/450 does use a compression release device and some have been known to break the return spring. Other things that can make it act that way are- starter may be going bad..or there a bad connection somewhere.. or...she has a valve timing problem IF you have load tested the battery and it tested good. Otherwise it's likely your spring is off, stretched or broken.