ZURICH (Reuters) - UPC, the Swiss arm of telecoms company Liberty Global and target of a 6.3 billion Swiss franc (5.35 billion pounds) takeover bid by Sunrise Communications , reported a narrower 3.6% fall in second-quarter revenue on Thursday.Investors are scrutinising UPC's numbers to see whether the Sunrise bid, to be funded by a rights issue, is properly valued. Sunrise's main shareholder, Germany's Freenet , has said it will not back the cash call.The drop in revenue at the mobile phone,..