LONDON (Reuters) – A deadline Thursday for the biggest shareholder in M&C Saatchi to clarify its intentions in making a bid for the advertising group is likely to be extended to allow the two sides to keep talking. M&C, one of the biggest names in British advertising due in part to its links to the ruling Conservative Party, has so far dismissed the advances of its biggest shareholder, an acquisition vehicle led by software entrepreneur Vin Murria. But a person familiar with the situation said the two sides were talking, meaning a deadline set by the Takeover Panel is likely to be extended. Mu…