A quaternary ammonium compound of formula (I): wherein R0, R1, R2 and R3 is each independently an optionally substituted hydrocarbyl group, X is a linking group, R4 is an optionally substituted alkylene group, 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.