The tent has been decorated and the invitations have been sent out for two very special Christmas episodes of "The Great British Bake Off." Eight bakers of "Bake Off" past return to the tent to compete once more. In this first episode there's another chance to see Mary-Anne from Series 2, Ali from Series 4, Cathryn from Series 3 and Norman from Series 5.The bakers face three festive challenges, starting with the signature challenge. Paul and Mary want them to make some highly decorative Christmas bakes that require intricate icing.For the technical challenge, Mary has a special present of mystery ingredients and a recipe which, if the bakers follow correctly, will produce a stunning bake, decorated with chocolate snowflakes.And finally there is the showstopper, for which the bakers must produce a festive masterpiece with Christmas theme.