It was the largest M&A cash bid ever to take place in the UK, and the firm that secured the role acting for the bidder shocked the City.

Simmons & Simmons acted for Telefonica on its record-breaking £17.7bn takeover of O2 last year, a deal that took just 10 days from start to finish.

Simmons secured the instruction thanks to the relationship of its Madrid office head Luis Felipe Castresana with the Spanish telecom giant. Castresana led the team on the multibillion-pound deal, a role that was won thanks to his previous advice to Telefonica on its purchase of a €240m (£164.5m) stake in China Netcom.

Castresana joined Simmons from the public sector as the managing partner of the Madrid office in 2000. He was previously chief counsel at Sociedad Estatal de Participaciones Industriales, the Spanish state-owned holding company.