Slaughter and May and Herbert Smith were in pole position in the £872 million Welsh Water bid for Welsh regional electricity company Swalec.
The bid was delayed to coincide with the latest valuation of the National Grid.
Swalec is one of the 12 regional electricity company (REC) shareholders in the grid, the sale of which will provide extra dividend for shareholders.
Corporate partner Nigel Boardman led Slaughters' bid team, which included partners Martin Whelton, Tony Beare (tax) and Laura Carstensen (competition).
Also involved was Welsh Water's in-house lawyer, company secretary Geoff Williams.
Boardman said of the deal: “The mechanical difficulties relate to the grid, and the company going ex-grid. Factors included how you value the bid.”
The team worked on the assumption that the grid float went ahead, he added.
Other top City firms involved in the deal include Linklaters & Paines for Natwest Markets, which advised the bidder.
Terms of the recommended offer, involving one Welsh Water ordinary share and £40.25 in cash for every five Swalec shares (equivalent to 940p net), were announced last week.