Examples of groups X2 or X1 include ???SiOH3, ???(Ra)SiOH2, ???(Ra)2SiOH, ???RaSi(ORb)2, ???Si(ORb)3, ???Ra 2SiORb or ???Ra 2Si???Rc???SiRd p(ORb)3???p where each Ra independently represents a monovalent hydrocarbyl group, for example, an alkyl group, in particular having from 1 to 8 carbon atoms, (and is preferably methyl); each Rb and Rd group is independently an alkyl or alkoxy group in which t