* Cosmen-led bid deadline to be extended again-sources
* Due diligence to take total of four weeks
* National Express shares up 0.25 percent by 1210 GMT
(Adds details, shares, background)
By Rhys Jones
LONDON, Sep 22 (Reuters) - The deadline for the Cosmen-led consortium to make a formal offer for National Express Plc is set to be extended for a second time so due diligence can be completed, sources close to the deal said.
The UK Takeover Panel had set an extended deadline of 1600 GMT on Sept. 25 for Spain's Cosmen family and private equity partner CVC Capital Partners to make a formal 500 pence-a-share offer for the British transport group or walk away.
But that now looks set to be extended further subject to the consortium confirming it intends to proceed with the 765 million pounds ($1.3 billion) bid, sources said.
The consortium led by Cosmen, which is the biggest shareholder in National Express with an 18.5 percent stake, believes it needs a total of four weeks to complete due diligence, which started two weeks ago, said one of the sources.
The UK Takeover Panel and National Express declined to comment. National Express on Sept. 11 said it had allowed the consortium to undertake a level of due diligence to better enable it to make a formal offer.
Shares in National Express, which have risen by more than 60 percent since it rejected a takeover offer from FirstGroup at the end of June, were 0.25 percent up at 475.8 pence by 1210 GMT, valuing the group at around 730 million pounds.
If the Cosmen-led consortium fails to make an offer it has promised National Express it will subscribe to a multi-million pound rights issue by the transport group, one source said.
National Express became a takeover target earlier this year after struggling to lower a debt pile of almost 1 billion pounds and announcing it would walk away from its flagship East Coast UK rail franchise due to mounting losses.
The Cosmen/CVC consortium improved its offer earlier in September after a 450 pence per share approach was rejected and also struck a side deal to sell National Express' UK bus and rail operations to rival Stagecoach. (Editing by Paul Hoskins and Jon Loades-Carter) ($1=.6082 Pound)