Audience measurement firm Taylor Nelson Sofres is pressing ahead with its merger with its German counterpart GfK, but it has nevertheless agreed to open its books to spurned suitor WPP Group, Reuters reports. TNS has already rebuffed WPP’s offer twice. Sir Martin Sorrell, WPP’s CEO, has engaged in a war of words with TNS, and has demanded that it be shown the same financial information that was provided to GfK. TNS and GfK agreed a nil-premium 50-50 merger that would create the world’s second largest market research outfit after Nielsen; the deal is valued at just under £2 billion. Sorrell is widely expected to up his previous 241.5p (£996 million) bid, with at least one analyst saying WPP could go as high as 300p.