Surely it would depend on whether or not the land is privately owned? So if the landowners at Great Park restricted access to 'pass holders' or contractors (in this case caterers, GNE etc), then Arriva couldn't access the site.

If Arriva feel as though they can operate the service on a commercial basis, then from GNE's point of view, it will boil down to the terms of the contract i.e have GNE been paid up front to deliver the service? Are GNE taking a percentage of all fares? Or is there something where it stiplulates GNE have sole operator access to the site etc.
I can see what your saying about the contract, but surely it is down to the terms within that.

Is it any different to another operator running something to Rainton Bridge? Yes GNE have the contracted 921, 922 and 923, as well as operating the commercial 20a and 35a, but if someone else wishes to run a service to the Nexus stops on a public highway, then I dont understand the problems in them doing so.

Looking at DCC axing the funding for tendered/contracted routes. If GNE/ANE chose to operate those routes commercially, who could stop them?
The contract holder has ended the contract (just as it did with the ANE x40) and it is left to the operators discretion what they then do - in the case of the DCC work, the operators didnt do much. In the case of the Great Park contract, Arriva have taken the decision to operate it themselves.