LONDON (Reuters) - At an English country mansion last month, lawyers for Royal Bank of Scotland (RBS) sat down with representatives of angry shareholders to broker an end to what may end up being the costliest case in British legal history.The meeting at The Grove, an 18-century estate near London that served as the secret World War Two HQ for Britain's biggest railway company, was convened to persuade investors to drop claims they were misled into stumping up 12 billion pounds ($16 billion) jus