When you roast maincrop potatoes, it is worth parboiling them until the surfaces are soft and can be roughed up, and placing them in hot fat to ensure crispiness. (There are further thoughts about roast potatoes in various postings, including this one.) Waxy varieties do not require this treatment.
I still like to parboil them, but only briefly, to remove some of the surface starch and to stop them sticking to the roasting tin. (Non-parboiled roast potatoes tend to have tough coatings.) I slice them, crosswise or lengthways, into pieces about the thickness of two £1 coins. I put them in cold water, bring them to the boil, and allow them to simmer for just a minute or so. I drain them and let them steam, before tossing them in olive oil (straight from the bottle) and roasting them. Usually, gas mark 6/200C for one hour, with a turn of the potatoes half way through, works fine; but it is always worth checking to see whether they are browning too quickly.
The Charlotte potatoes I buy in France respond particularly well to this treatment. The more thinly sliced ones emerge from the oven with the texture and flavour of high-class crisps.