Thank you for your quick response - I haven't made an SAR request, but intend to and hadn't thought of sending an SAR to M & S, but will also look into this, nor have I tried to access my previous NatWest Bank Account records, but will also approach them (I did a bit of "online" research and it appears they only keep records for 6 years).
I'm fairly sure I took the card out in 1989 and paid it off in 1996 (as per Santander's records). I think I owed approximately £2,500-3,000 and only ever paid the minimum payment until redemption.
They've based the award on a balance outstanding as at July 1996 being 15p.
I will most definitely take the attitude "I will... " rather than "I doubt..." from now on.