In an exclusive interview, Stephen Sackur speaks to president of Ukraine Petro Poroshenko, who has been in Britain to attend a summit of NATO leaders. There is a consensus view that the crisis in eastern Ukraine represents the most serious threat to Europe's security and stability since the fall of the Berlin Wall - Ukraine and Russia are just a few steps away from all-out war, but right now there are hopes of a ceasefire. So, is there a path back from the brink?