Not just one day...
It wasn't just Monday - there were service issues all weekend too.
I couldn't logon on Sunday, online or in app, and the Secure Message service (what you need to use to complain if you don't want to listen to hold music) was borked from Friday evening to my certain knowledge.
Had a long conversation with specialist complaints today about other issues - overly aggressive fraud prevention measures repeatedly stopping me making online payments to people like Amazon, the TrainLine, major retailers, and the fraud department diverting to a call centre that closed at 8PM - which was what I was trying to complain about on Friday having had more payments blocked. Decided following that conversation ("we can't change this because it's a shared service with other providers") that after Christmas it's time to go and talk to another bank :-( Apparently 24/7 banking means something different to Barclays and the rest of the world...