I have found why the screen changes, and as the reason may affect other Boeworld members, I am detailing the reason.
My 7-40 Mk II has the usual DVi-D adaptor, as will any tv which only has a DVI input. This adaptor, a DVI-D single link, is ok for a Sky HD box. The new 1Tb box, however, has a greater band width due to its capability of transmitting 3D images. So this later box requires a DVI-D dual link adaptor. Fitting this (it's about£1) has solved the problem.