“…maybe a private message to them via their facebook page might get more information?”
Personally, if the circumstances are as you describe, I would start with this man:
Patrick Bousquet-Chavanne
Executive Director of Customer, Marketing and MS.com
Marks and Spencer Group plc
Waterside House
35 North Wharf Road
London
W2 1NW
I find you usually get better results if you avoid the CEO but choose somebody at board level, so leave Steve Rowe, the CEO, alone. Your enquiry will still almost certainly be dealt with by a lackey, but you should get a better response. You might ask them whether this is the way they treat customers of 80-odd years old and whether they treat all suspected thieves in that way, regardless of the circumstances. Methinks an over-zealous (and idiotic) store manager may be the cause of this.