Angie
You have to make an appointment at the Police Station in Paleokastritsa. An appointment is necessary as they only have one officer who deals with the applications for a resident permit.
Cost is, I think 1e and they are now issued for life.
Allow at least 2 hours as they complete all paperwork there and you leave, hopefully, with a permit.
Take 4 copies of a recent passport photo. There is a shop in Acharavi who can take these.
Best to have 4 copies of everything. - Passport , Tax number , Bank book and I think a recent utility bill, showing a UK address.
Sandy is the latest recipient that I know of who has a resident permit so she will know more.
The officials do tend to change the "rules" occasionally on what paperwork is required.
I am sure that your accountant will soon tell you that, whatever local office you visit, take 4 copies of all your paperwork. (Even your resident permit, when you get it)
Cheers
Negg