Amos and Mosha may have been acquired for Whipsnade all along, as London already had Sam and Sally on the terraces at the time,
I believe it was mentioned somewhere on the Forum that one of the pairs (Amos and Mosa?) were a political gift from(was it?) Russia. So were not deiberately obtained but rather were accepted when they didn' really need them- or maybe they thought they could place them at Whipsnade anyway.
I thought the Enclosures at both ends of the Mappin's bear range had 'large' pools and the central ones smaller pools which couldn't really be seen from the visitor walkway.