From memory:
J = fully flexible, fully refundable
C = semi flexible - £30 change fee (or is it £40 now?) - Non refundable Seat pre-asignment possible
D = semi flexible £30 change fee (or is it £40 now?) Non refundable
R = non flexible, non refundable.
I = non flexible, non refundable. Sold for CW and in CE as part of long haul trips only