There is provided a compound of the formula (I) wherein R1 is an alkyl, alkenyl or alkynyl group containing x carbon atoms, wherein x is from 1 to 10, wherein one of R2 and R3 is an alkyl, alkenyl or alkynyl group containing y carbon atoms, wherein y is from 1 to 10, wherein the other of R2 and R3 is a branched group of the formula (II) wherein q is from 0 to 3, wherein each R5 is independently selected from —OH and —O—C(O)—R4, wherein n is from 10 to 20 and m is selected from 2n-q, 2n-2-q, 2n-4-q, and 2n-6-q, wherein each R4 is independently selected from alkyl, alkenyl and alkynyl groups containing z carbon atoms, wherein z is from 7 to 21, wherein z is different to at least one of x and y.