Gave someone my details to setup a direct debit earlier so they could pay money IN each month and they said they may need a bic number because they had never sent money to an online bank before
Bank ID number
It’s in the app, on your profile
MONZGB21
Can’t see why they need it though 
Also known as a SWIFT Address or SWIFT code
It’s good that whoever it was is anticipating potential problems for something they’ve never done, rather than just saying no. 
A direct debit to pay money into your account each month? That wouldn’t be a debit but a credit right?
Can direct debits work in the reverse direction? I’ve never personally experienced this
Whatever it is, it will be some form of bank transfer every month 
Direct Credits are what business typically use to pay salaries. It’ll likely be a Faster Payment or something along those lines.