Does DD setup all happen electronically or is there some snail mail involved?
Depends.
Original system - Customer fills in application form and posts back to company. Company posts application to the bank.
AUDDIS - Customer fills in application form and sends back to company. Company submits application electronically to the bank through BACS.
Paperless - Customer signs up on line or on phone. Company submits application electronically to the bank through BACS.
So, no snail mail for Plusnet, or most organisations these days I imagine. Can't remember when I last had to fill in and post a paper DD application.