" camera is only a few weeks old and primary test lens is a Sigma 50MM prime F/1.4"
Try your tests with a Nikon lens. If you don't have a fast Nikkor, then borrow one or try with a demo lens at a local shop. Some 3rd party lenses have problems with newer camera models. If the problem is with the lens, Sigma should be able and willing to update the lens firmware.
"i have no idea if the local NIKON service centres have much experience (or the correct equipment) to service the D800."
Right now in the US, Nikon USA will not allow authorized repair centers to service a D800/D800e.
Gary in SE Michigan, USA. Co-organizer of the Southern Michigan Chapter Nikonians membership - My most important photographic investment, after the camera. D4, D800e, D300, D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery