Have only just spotted this thread and whilst it's pretty old right now, I thought I'd add my suggestion - for what it's worth!
The best roast tatties are made with 'floury' potatoes and it's essential to remove their skins. Cut them to the size you require, then, as suggested by several people, parboil them. Say 5-6 mins, just so the outer quarter of an inch or so is softening. Drain the water away - and here's the magic - put the lid back on and shake the pan until the outer edges of the potatoes are roughed up. This ensures that the outside becomes really crunchy.
For me the next ingredient is essential - duck or goose fat. Melt the fat in a foil container and when hot, add the potatoes and move them around until they are all coated. Cook indirect hot. Add rosemary sprigs, but remove if/when they start to burn/blacken.
Eh voila - fluffy middles and crunchy skins - divine!