Just now: I logged in and clicked the red "Manage an Existing Booking" option from the list on the right hand side. Was offered just a "enter your booking reference and name" rather than having my upcoming trip already populated (as it was last week). But if I click "Manage" from the top menu, the drop-down box has my upcoming trip listed.

In going back to check this (while typing the previous paragraph) so I could describe it accurately, it's logged me out. Clearly something buggy going on.