* UK Takeover Panel sets Sept 11 bid deadline
* National Express says still seeking clarity on offer
* National Express shares up 2.4 percent (Adds National Express comment, background, shares)
By John Bowker and Victoria Bryan
LONDON, Aug 5 (Reuters) - National Express bidders the Cosmen family, CVC and rival Stagecoach have until 11 September to make a bid for the troubled rail and bus operator, the UK takeover regulator said on Wednesday. The Cosmen family, which owns 18.6 percent of National Express, last month made an all-cash proposal for National Express in conjunction with private equity group CVC.
Stagecoach then stepped into the fray by saying it was in talks with the Cosmen consortium about taking parts of National Express if a bid was successful, while also considering a bid of its own.
National Express shares were up 2.2 percent by 1358 GMT, while Stagecoach was up 1.7 percent.
A spokesman for National Express -- which insists it has a future as an independent company -- said it welcomed the new deadline.
"The board continues to seek clarity from the consortium about a possible offer, which at present is complex and subject to a number of conditions," he said.
"The group is working on a range of measures to strengthen both the operations and balance sheet," he added.
National Express, which runs bus operations in Spain and North America, has become a takeover candidate after amassing a debt pile of close to 1 billion pounds ($1.70 billion), giving up its flagship UK rail franchise and announcing the departure of its CEO.
It said last week it had cut its debt by 200 million pounds to 977.4 million via measures such as scrapping the half-year dividend. For a column on National Express, see (Editing by Rupert Winchester)