Listed advertising and marketing services firm STW Communications has gone into a trading halt amid speculation of a takeover by Sir Martin Sorrell’s WPP. The move by WPP is not unexpected with the two businesses being closely associated since the late 1990s. STW and WPP partner in some businesses such as Ogilvy, and WPP already owns 23.6 per cent of STW through its subsidiary, Cavendish Square Holdings.