A compound of formula (1a), (1b) or (1c) wherein: n is 1 or 2; RN is H or Me; R1 is optionally one or more halo or methyl groups; R2a and R2b are independently selected from the group consisting of: (i) F; (ii) H; (iii) Me; and (iv) CH2OH; R2c and R2d (if present) are independently selected from the group consisting of: (i) F; (ii) H; (iii) Me; and (iv) CH2OH; R3a and R3b are independently selected from H and Me; R4a is selected from OH, —NH2, —C(═O)NH2, and —CH2OH; R4b is either H or Me; R5 is either H or Me; A is either (i) optionally substituted phenyl; (ii) optionally substituted naphthyl; or (iii) optionally substituted C5-12 heteroaryl.