The invention provides compounds of the formula: R1 is OR11, or NR5R5′; R2 is H or F; R5 is H, C1-C6alkyl, OH, C(═O)R6, O(C═O)R6 or O(C═O)OR6; R5′ is H or C1-C6alkyl; R6 is C1-C6alkyl or C3-C7cycloalkyl; R13 is H, phenyl, pyridyl, benzyl, indolyl or naphthyl wherein the phenyl, pyridyl, benzyl, indolyl and naphthyl is optionally substituted with 1, 2 or 3 R22; and the other variables are as defined in the claims, which are of use in the treatment of cancer, and related aspects.