Just got an email update from TFL
"We are sorry that it is taking longer than anticipated to establish the new signalling system between Finchley Road and Euston Square. As part of our commitment to safety, drivers must be accompanied through the new section by an instructor while it beds in. This means that fewer trains than normal are running.
We are working to restore a full service as quickly as possible. Please allow longer for your journey and visit our
status updates page for the latest information.
Customers who have experienced delayed journeys will be automatically refunded."