I’ve had a search but couldn’t find anything, the master card UK website lists various benefits such as:
Doubles the original manufacturer’s or store brand warranty for up to one year when you pay with your eligible Mastercard.
Should you find a lower price for a new item within 60 days from the date of purchase using your eligible Mastercard, you may be reimbursed for the price difference.
Provides coverage for most items you purchase with your eligible Mastercard if the item is damaged or stolen within 90 days of the date of purchase.
It also states:
To learn about the benefits available with your credit or debit Mastercard, call Mastercard (0800-96-4767) or contact your issuing financial institution.
The webpage is here: https://www.mastercard.co.uk/en-gb/consumers/features-benefits/card-benefits.html
So what, if any, benefits does having a Monzo Mastercard Debit have?