Yes The EU-27 need an unanimous decision. One single EU country can stop the extension of the Art 50 period beyond March 29. That's why I think it is important that the UK can decide unilaterally to revoke Art 50. If May would make clear to the EU that she would do that, only to retrigger Art 50 after the new referendum, when that referendum decides no-deal or the May-deal, I think the EU countries that would vote against extension of Art 50, could choose to be persuaded. Choose... Because strictly considered, keeping the UK sovereign in its responsibility to handle its own choices, including burning all that time, keeping itself as EU out of domestic UK political issues, the EU could choose as well to let that revoking happen. A bit the same as the argument above: Why would the EU interfere with domestic and sovereign UK political choices on whether that referendum would exclude any of the 3 possibilities: no-deal, May-deal, Remain ? If the EU would exclude no-deal as condition for that Art 50 extension, the hardcore Leavers will blame the EU forever that they are imprisoned. Cameron, May as PM, the Tories had a big mouth about taking back control... well... let them have it... including the responsibility of the referendum choice on two or three questions.