Hi Darius,
Thanks for the quick response! The info you requested is as follows;
Model identifier: MacBookPro5,3
My system disk is a 120gb Apple SSD (APPLE SSD TS128A Media), and the additional disk is a Seagate Momentus 7200.5 ST9750420AS.
The Apple SSD is in its original location, and the Segate sits in the Optibay.
I've just disabled SMS, and from an initial test there seems to be no change in performance.
Please let me know if there's any other tests you'd like me to do - happy to help as I'm keen to get this fixed! 
Cheers,
Ross