A compound of formula Ia, Ib, Ic or Id: 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; m is 1 or 2; q is 0 or 1; R11a, R11b, R11c and R11d are independently selected from H, halo, C1-4 alkyl, C1-4 fluoroalkyl, C3-4 cycloalkyl, C1-4 alkyloxy, NH—C1-4 alkyl and cyano; R12a and R12b are independently selected from the group consisting of: (i) F; (ii) H; (iii) Me; and (iv) CH2OH; R12c and R12d are independently selected from the group consisting of: (i) F; (ii) H; (iii) Me; and (iv) CH2OH; R12e is H or Me; R13a and R13b are independently selected from H and Me; R14 is either H or Me; R16a and R16b are independently selected from H and Me; R6 is selected from H, OMe, and OEt.