Mostsuitably X is an oxygen atom and A.sub.2 is a methyl or t-butyl group and A.sub.4 is a phenylene group.A further particularly suitable sub-group of esters of formulae (VIII) or (IX) are those wherein R is a group R.sup.1 or CHR.sup.2 R.sup.3 wherein R.sup.1 is a hydrocarbon group of 1-9 carbon atoms optionally substituted by halogen, lower alkoxy,lower acyl, hydroxy or lower acyloxy groups and