Thanks for the reply! I did a quick test file and it comes out as yours with Area showing one face, Exposed Area showing both. In the live file that they're working in, using surface area or exposed area didn't make any difference, however on further investigation it turns out they'd exploded cladding panels into morphs, so every panel actually had a 3mm thickness and so Area was picking up the sides & back too...!!
Lesson for today, always check the number of morphs..! Thanks for your time 🙂