U.S. private equity firm Blackstone (BX.N) has joined forces with buyout fund Hellman & Friedman to bid for British payments processing company Worldpay, two sources familiar with the matter said on Thursday. Worldpay’s private equity owners, Advent and Bain, are preparing the company for a London stock market listing which is likely to propel the firm into the FTSE 100 (.FTSE). Last month, they appointed Barclays’ deputy chairman Michael Rake as Worldpay’s new chairman, replacing John Allan.