Springy Rattling noise on new bike
So eventually it seems I figured it out!

The chain was indeed vibrating like a guitar string at speed. This was caused by a couple of rear gears ever so slightly out of alignment. They shifted ok but I could see the chain was marginally catching on some rear cogs. 

A small adjustment of the gears with the dial on the gear shift and a good grease of the chain and it was running silent :)