Thanks everyone. In the end, the ganache didn't set. At all. It's just chocolate sauce, so I need to find a use for that . The Vienna cream was fine, and I chilled the cakes sufficiently to halve them, and packed it all (the cakes and Vienna cream, not the ganache) in a cold box, to assemble on site. I also bought two filled sponges, but not a crumb of cake was eaten, so it was all for nought. I wasted my day.