Why are my roast potatoes not coming off crunchy?
Your result looks like too much water, and too high a roasting temperature for an even, golden crust. (The ones illustrated in the recipe don't look as if they achieve that, either).

My way: Choose a floury, not a waxy, potato variety.

Par-boil from cold in salted water, until 3/4 cooked, a skewer should meet resistance before reaching the middle. (about 9-10 minutes, your size.). Drain in a colander, and leave to dry completely, but not cool. 

Pre-heat your fat in the roasting tray, a thin layer. Transfer the potatoes to half-fill a dry bowl, pour over the hot fat, and shake gently to break up the surface. The roasting tray should be big enough to give the potatoes room to breathe, they shouldn't be crammed in. 

Roast at about 180C (fan) for 40 minutes or so, turning once or twice. If your oven gets steamy, open the door a crack every now and then to let the steam out. Before serving, keep warm, uncovered, somewhere that doesn't trap steam, on paper towels to wick away excess fat.