It will be appreciated that the hydroxyalkylating agent is selected to introduce either a 2-hydroxyethyl group or a 2,3-dihydroxypropyl group into the compound of formula II. Thus, for example, the compound of formula III preferably used is a reactive ester derivative such as a compound of formula III in which X represents a halogen atom e.g. a chlorine or bromine atom, or a sulphate or hydrocarbo