Changing wheels on Scott 960 - will 29" 12 x 142 fit I've recently bought a set of wheels I'm hoping to change over to for a Scott 960 (2014).

The specs of the bike are here https://www.vitalmtb.com/product/guide/Bikes,3/Scott/Scale-960,13562 

I wasn't sure from these specs how to find what the correct hub size should be for the wheels. 

I've bought the following wheel set:

Shimano SLX MT66 Wheelset Black 29" 12 x 142 Shimano


A friend of mine thought the original wheels may be 29" 10 x 135 rather than 12 x 142. How from those specs would I determine this and does this mean I need to return the wheels?
I think you are out of luck and have ordered the wrong size rear wheel.

Looking at the page you provided a link for, the bike has a quick-release rear hub. (I had to open the picture in a separate tab and zoom in to determine this, the text does not appear to specify the hub width or type). 

The wheelset you have ordered has a 12mm thru-axle, so regardless of the hub width it will not work.

Have a look at the Terminology Index and search for 'quick release' and through axle' if you are not familiar with the difference.

It's very easy to determine the hub width you need. Just get the rear wheel out and measure the distance between the inside faces of the drop outs with a ruler.