Synthesis and structural characterisation of the trinuclear barium disiloxanes [Ba3(OSiPh2OSiPh2O)3(tetraglyme)2] and [Ba3(OSiPh2OSiPh2O)3(hmpa)5(H2O)], containing an unusual Ba3O7 core [tetraglyme = Me(OCH2CH2)4OMe; hmpa =(Me2N)3PO]