I’d like to request that we hold off on reporting Morrisons to Mastercard for now. 
In general, please report merchants to us before Mastercard. In almost all cases, we can be more friendly and get these situations resolved quicker. Acceptance issues are sometimes fairly complex and the merchant is not always to blame. 
We have already been in productive talks with Morrisons’ payments processor and as far as I am aware, the new whitelist is rolling out. Due to the nature of their specific setup and procedures, this does take a long time though. The post I’ve linked below provides a time frame of January 2018 for Monzo cards to be supported at Morrisons Pay at Pump systems. 
https://community.monzo.com/t/pay-at-pump-fuel/15174/52?u=richardr