Bike insurance - what value do you insure for?

So, the child has just been treated to a very nice but secondhand Nukeproof Megawatt. I'm not at all jealous as it's a far nicer bike than I've ever bought myself but hey...

Looking to insure it and, despite my years of commercial underwriting experience, am a bit confused as to whether we declare the secondhand value or the new value. 

I'm very happy to insure it for what we paid for it however I don't want the insurer to then apply averaging to any claim if they feel it was underinsured. 

Can anyone help me out?