THE ARCHAEOLOGY

The Saltmarsh and its henge are both fictional. However, a Bronze Age henge was discovered in Norfolk, at Holme-next-the -Sea.
http://en.wikipedia.org/wiki/Seahenge

Francis Pryor has written a wonderful book about Seahenge and its discovery - Seahenge: A Quest for Life and Death in Bronze Age Britain
http://www.amazon.co.uk/Seahenge-Quest-Death-Bronze-Britain/dp/0007101929/ref=sr_1_1?ie=UTF8&s=books&qid=1223284919&sr=8-1

Ruth discovers an Iron Age bog body. One of the most famous bog bodies is Lindow Man. You can see Lindow Man at the British Museum.
http://www.mummytombs.com/bog/lindow.htm

Ruth finds an Iron Age torque buried with the body. A famous Iron Age treasure hoard was found at Snettisham in Norfolk in 1950.
http://en.wikipedia.org/wiki/Snettisham_Hoard
http://www.compulink.co.uk/~archaeology/hilites/snet.htm

For more about Celtic torques http://www.colchestertreasurehunting.co.uk/torcs.htm