First thing is timing. Start this process an hour before the roast is going to come out:
Put lots of fat in your baking tray. Use bacon fat or duck fat if you have it (I collect the fat whenever I cook bacon in a pan, just for this kind of thing), and top up with sunflower oil. If in doubt, use more rather than less. Put the tray in the oven to warm up.
Boil the potatoes for 5-6 minutes, then up-end them into a colander. Put the pan lid on top of that, hold on to the lid, and give it all a good shake. This makes the surface of the potato rough and helps the fat get in there. (If you have too many for the colander then do this with the lid on top of the pan that you cooked the potatoes in.)
Take the tray out of the oven and quickly transfer the potatoes in there with a couple of spoons. It's important to keep the fat hot, so you could even consider putting a gas burner on low underneath the tray. Try to keep the flat sides of the sliced potatoes in the air at this stage, as the flat bit is more likely to stick to the base of the pan until it's cooked for a while. Spoon some of the fat over the potatoes.
Put the tray in the highest part of the oven. After 25-30 minutes, take the potatoes out of the over and turn them over. Give them another 15 minutes while the roast finishes.
When you take the roast out to rest, whack the oven temperature up to 220° and give the potatoes 10 or 15 minutes to crisp up.