Compounds of formula (I) or a salt or solvate thereof, wherein: x represents 0 or 1; y represents 0 or 1; R1 and R2 independently represent —C1-9 alkyleneC1-6 fluoroalkyl, which fluoroalkyl moiety contains at least 1 fluorine atom and not more than 3 consecutive perfluorocarbon atoms and wherein the R1 and/or R2 moiety is optionally interrupted by an ether link, processes for their preparation, use of the compounds in the preparation of pharmaceutical formulations and the formulations are described.