Engine is 1.6tdci duratorq fitted to 2013 Ford Mondeo

Crank pulley spun itself off, bolt still attached and whole. Photo's show extent of damage. 

I have fitted a new crank pulley, new bolt and loads of thread lock. How long will this likely last? Is it possible to repair or will it require a new crank or engine? My worry is the threads inside the crank are worn given the spinning nature of the bolt however it took the new one and tightened no problems.

If it tightened ok then I reckon you have nothing to worry about assuming correct torque procedure followed. Very lucky no other damage.

The Crankshaft should be harder than the bolt otherwise the bolt would not stretch, so it should be ok.

The only thing I would have done differently would be to use a Tap to clean out any old Thread locking compound that someone may have used from the Crankshaft. You would be surprised by how much can come out and if it is not removed then the same thing may happen again.

If you don't have a Tap then you can make an improvised one by taking a Hacksaw to an old Bolt at right angles to the thread at 4 points, then file any burrs away. This is something that I always do with Suspension Bolts as well and works fine but it doesn't all came out in one go.

I would keep checking the Pulley and if there are any signs of loosening then try the above with a new Bolt.