PLAGIARISM or breach of copyright are difficult things to prove in a court of law because literary or other artistic ideas can plant themselves in someone's mind and remain there unnoticed and unused for years until they emerge in a reshaped or revised form that owes only a distant resemblance to the original. Sometimes a whole paragraph can be written by one writer without being aware that he or she had first read it in almost identical form in another book. In the case that opened in London's High Court yesterday the issue is somewhat clearer. The authors of The Holy Blood and the Holy Grail published 20 years ago claim that Dan Brown's astonishingly successful novel The Da Vinci Code is based on the “architecture” of their factual book and that the essential theory they developed after ten years of research forms the basis of Mr Brown's novel. Their ten million pound claim is presumably based on the extraordinary success of The Da Vinci Code which has become the world's best-selling hardback novel with sales of nearly 30 million. The stakes are very high, both in financial and literary terms. The film of The Da Vinci Code, starring Tom Hanks and Audrey Tautou, is due to open this year's Cannes Film Festival and to be released in Britain in May. For Dan Brown and the authors of Holy Blood, Holy Grail the issues are financial but also creative since the judgement will bear on how far an idea can be protected from use by others, even if their use of it was subconscious rather than deliberate. For the defendant publishers, Random House, there is a different dilemma; they brought out both books.