Virtually all such posts are advertised openly and competitively - certainly across Departments and other public bodies such as the Research Councils and increasingly in the scientific press. There are no large concentrations of disciplines but the range of disciplines is enormous.
Physical scientists work alongside engineers in construction and transport.
Geologists and soil scientists work with planners. Some areas of environment employ physical modellers alongside meteorologists, chemists and various biological disciplines. Biology, ecology, ecotoxicology, microbiology, marine biology, etc. You can, in fact, find almost any discipline in DETR from operational research to waste engineers, radiochemists and biotechnologists. A few scientists are recruited at the most junior levels with first degrees.