it is a quaternary ammonium compound of formula (i): in which r0, r1, r2 and r3 are each independently an optionally substituted hydrocarbyl group, x is a linking group, r4 is an optionally alkylene group substituted, r5 is hydrogen or an optionally substituted alkyl, alkenyl or aryl group, and n is 0 or a positive integer, provided that n is not 0 when r5 is hydrogen.