According to the invention there is provided a compound of Formula (I) in which: A is a C2-8 alkylene group; R1, R2, R3, and R4 are independently selected from hydrogen, C1-4 alkyl, C2-4 dihydroxyalkyl in which the carbon atom attached to the nitrogen atom does not carry a hydroxyl group and no carbon atom is substituted by two hydroxyl groups, or R2 and R3 together form a C2-6 alkylene group which with the nitrogen atom to which R2 and R3 are attached forms a heterocyclic ring; X1, X2 and X3 are independently selected from hydrogen, hydroxyl, NR1-A-NR2R3R4+(Zm−)1/m, halogeno amino, C1-4 alkoxy or C2-8 alkanoyloxy; and (Zm−)1/m is an anion of charge m; or a derivative in which the group NR1 is quaternarised.