You can still enter/exit via the side (on York Way) and move to the right to get to the Underground without going outside or into the new concourse. It's just straight ahead that is closed, including all the shops and toilets (which went some time ago). At least that was what it was like last Thursday (with staff handing out a map showing the new layout).
I am not sure how much room there will be when a few trains arrive close together, but I am sure they'll be encouraging people to exit to the right (from the platforms) to go into the new station area. Most commuters will quickly learn, I'm sure (after moaning like crazy for the first few days!).
There's big signs up saying to turn right after the gates - which is annoying as I assumed this meant York Way was close and I didn't come across anyone handing out maps (very half heartedly too!) until I was already near the Underground entrance.
I wonder why there was no notice this time - I passed through Wednesday evening too.
