Get fluffy potatoes (King Edwards are lovely but you have to deal with a lot of crumbling - it's like they mash themselves). I now think Maris Piper are the best; fluffy but they hold together
There's no big secret; really hot fat, parboiled potatoes and that's it
Method
Set the oven to 200-220 and peel the potatoes and if they are big, half them and leave the smaller ones whole. Try to get them all roughly the same size
Put them in a big pan of cold water and sprinkle a little bit of salt in
Bring to the boil and then turn it down to a gentle simmer. 5 minutes should so it but keep checking with a fork. Once the fork can easily get about a quarter of an inch in they are done.
Drain them and let them stand for a minute to dry out a bit
Put the lid back on the pan and while holding it on give it a really good shake to smash up the cooked outsides of the spuds. This will be the crispy bit
Get the really hot fat out of the oven and pour the spuds in (away from you in case it splashes)
Baste them all as quickly as you can - you don't want it all cooling down too much
Put them in the top shelf of the oven for around 40 minutes but it varies