Construction and engineering group Costain has returned to the table with what it terms 'a significantly enhanced proposal' to take over contractor Mouchel.The new offer is on the basis of 0.5947 Costain shares for each Mouchel share and, based on the closing price of Costain's shares on 5 January, values each Mouchel share at about 135p.Mouchel's shares closed at 107.75p on 5 January and were trading at 105.8p the day before Costain went public, on 22 December, with its interest in acquiring Mouchel.Under the enhanced proposal Mouchel's shareholders would control around 51.7% of the enlarged issued share capital of Costain. 'Mouchel's shareholders would, therefore, own an increased equity interest in a well capitalised, enlarged business, with a clear strategy for future profitable growth,' Costain said.'Our ability to formalise a proposal that is attractive to Mouchel's shareholders and other Mouchel stakeholders and is recommendable by the board of Mouchel is dependent upon gaining access to satisfactory due diligence in order to confirm our key assumptions, including those relating to synergies,' said Costain chairman, David Allvey.