You have to hand it to Putin, nobody can predict his next move. If his partial military withdrawal can kick start the Geneva peace conference then Putin will be flavour of the month and will demand his quid pro quo that the EU lifts its sanctions against Russia after his Ukraine adventures but I suspect Putin will also want an agreement that the EU does not intefere in Ukraine.
If this deal sticks then migrant flows will drastically reduce and save Dave's bacon. Cameron has what Napoleon demanded of his Marshals of France, he is lucky. Remember however the Turks and Russians are old enemies as are the Turks and Greeks in Cyprus which is an EU state.
I confidently expect the FCO will cock this one up.