I recently had this bit of info passed on to me by another member, andrewbladon, who was part of a group in the same situation. They hired a local taxi which took them from Didima all the way back to Sentinel Car Park, for which they paid R1500.
I am still awaiting a reply from Cathedral Peak Hotel as to whether they offer such a service, or would consider doing it in future. You may want to phone them yourself.
Another possibility would be to contact one of the other resorts or lodges in the Northern Berg that cater for foreign tourists that don't have their own transport (like the backpackers) - they may have a minibus for situations like this, especially since their foreign guests do hikes that have transport challenges.
