From my experience with electronics, I concur with Richards suggestion that the fault may be the same as last time and just not recognised properly in the previous fix. Taking this with the fact that it was a demo model (so obviously working in the shop), but failed as soon as you got it (after it was moved and re-installed in your system), a mechanical problem such as a dry joint on a wire or a problem with the volume pot or a relay are the obvious candidates - however this speculation is far from certainty.
The good news is that these types of problem are almost always an easy and complete fix when the problem is identified correctly, The bad news is that identification is tricky as the problem can sometimes just 'go away' because of mechanical vibration during transport.
As an ex-demo XS 2 it certainly won't need servicing yet, so just getting the repair done (and hopefully done fully and successfully this time) should see you get many years of good service from it. I'm sure if it's sent to Salisbury, then given the history of the unit (and your location), they'll be particularly diligent in ensuring a proper repair is done - everyone at Naim are proud of their product, take pride in their work and really do their best to ensure happy customers.