Neil Oliver goes in search of Bronze and Iron Age sites that were sacred to ancient Britons, with water seen not just as a source of life, but also of reverence. At Flag Fen near Peterborough he discovers a vast ancient causeway built across the fens, with sacred objects placed among its timbers. Neil travels to Anglesey, where swords, precious artefacts and even a slave chain were ritually deposited. Moving on to Bath and its sacred spring, Neil discovers an early version of the habit of throwing coins into water. Once here the Romans recognised the old gods but also brought their own too, making Bath one of the most sacred sites in Roman Britain.

• Neil Oliver