They appear to share all Three and T-Mobile sites but strategic Orange sites are excluded.
We live on a development that is on the side of a hill. The signal for all networks is non existent (no matter what their coverage checkers say) appart from EE.
There is an Orange mast opposite the development, I was hoping it would be added to the sharing agreement after the roaming agreement with Orange ended. It didn't. 2000 homes coveted by only one mast on one network. They were never going to share that!
I'm on Three for my mobile, and T-Mobile for my Nexus 7. I only stayed with Three because they give me a HomeSignal box otherwise it would have had to be a move to T-Mobile or Orange.