each group R independently is alkyl or cross-linked alkenyl, (preferably C1-6 alkyl or cross-linked C1-6 alkenyl, in particular methyl), or cross-linking oxy, and each group A is a group as defined for R (with the same preferred groups as for R) or a side-chain (as hereinbefore defined) comprising an end-capped polyether or polyether ester, eg a capped polyalkylene oxide or polyalkylene oxide carb