I decided to take the recommendation here and went with Elite Parking Services ... it went seamlessly. They have a continual presence of staff in orange jackets. Car got its free wash, wasn't damaged, was ready on-time and documents were on-point. They're not the cheapest (R75/day for 6 days), but it was only marginally more expensive than OR Tambo's own long-term parking and it felt more convenient and secure.
Thanks just booked with them, ratings online look good and their price was the same as the previous company I used. Also looked at the long term parking but it was full so this way at least get a free wash