Potato parts in the oven
That was bad advice. If you're not par-boiling the potatoes they will need at least 40 minutes, but to be honest you are much better off par-boiling.

Pre-heat the oven as before and place a roasting tin in to pre-heat as well.

Cut the potatoes as before, then place them in a large pan of salted water, bring it to the boil, and simmer for 5 minutes. Drain the pan and place it back on the heat for a little while to drive off the moisture in the pan and on the potatoes.

Transfer the potatoes to a bowl and drizzle generously with oil. Season well (you can add spices, smoked paprika is great), mix through, then place in the hot roasting tray and roast for 30-35 minutes. 

The boiling process begins to cook and soften the potatoes, and then the dry heat of the oven carries on cooking them while also crisping the outside.