The invention provides compounds of the formula (I): or salts, tautomers, solvates and N-oxides thereof; wherein R1 is hydroxy or hydrogen; R2 is hydroxy; methoxy or hydrogen; provided that at least one of R1 and R2 is hydroxy; R3 is selected from hydrogen; halogen; cyano; optionally substituted C1-5 hydrocarbyl and optionally substituted C1-5 hydrocarbyloxy; R4 is selected from hydrogen; a group —(O)n—R7 where n is 0 or 1 and R7 is an optionally substituted acyclic C1-5 hydrocarbyl group or a monocyclic carbocyclic or heterocyclic group having 3 to 7 ring members; halogen; cyano; hydroxy; amino; and optionally substituted mono- or di-C1-5 hydrocarbyl-amino; or R3 and R4 together form a monocyclic carbocyclic or heterocyclic ring of 5 to 7 ring members; and NR5R6 forms an optionally substituted bicyclic heterocyclic group having 8 to 12 ring members of which up to 5 ring members are heteroatoms selected from oxygen, nitrogen and sulphur. The compounds have activity as Hsp90 inhibitors.