WPP has bought a 49.9pc stake in Clemmow Hornby Inge, the London-based marketing firm whose clients include Argos, British Gas and Carphone Warehouse. The amount paid was not disclosed, but CHI's worth was estimated at £60m last year, when it had sales of £18.5m and gross assets of £17.7m. The acqusition adds another creative string to WPP's bow, which already owns JWT and Ogilvy & Mather. CHI will retain its independent positioning. WPP chief executive Sir Martin Sorrell has predicted a bumper couple of years for advertising as clients capitalise on the Beijing Olympics, the US presidential election and the European Championship football. He expects worldwide advertising spending to grow 4pc this year and possibly more in 2008, though he remains concerned about 2009 due to structural problems in the US economy. CHI is a specialist in the UK, working across all media - from traditional print and television to the internet. Under the creative leadership of Charles Inge, CHI was responsible for the signature "Mobly" campaign for Carphone Warehouse and revitalised the Tango campaign inherited from another agency. The business started in 2001 with the Carphone Warehouse as its inaugural customer and has since built a roster of 26 major clients that include British Gas, Royal Bank of Scotland, Teletext, Toyota and the Telegraph Media Group. It has so far resisted expansion overseas but now employs 150 people in the UK. Observers say the agency's success is down to the relationship between the founders. Simon Clemmow is considered a commensurate strategist, Johnny Hornby a compelling front man and Mr Inge one of the leading creatives in the industry. CHI won Marketing magazine's creative agency of the year in 2004. The company is a fierce rival of Bartle Bogle Hegarty, from whom it has often poached and to whom it has lost staff. Nielsen Media Research calculated that its billings reached £135m in 2005 - up from £88m in 2004, the most recent data shows.