Okay, so I got to Alfthand and made my way through the glacial ruins to Alfthand Cathedral. There's a lift inside the Cathedral itself which takes you back up to the surface of Alfthand. I did this, but I can't get the gate to open - there's no action on the gate at all to allow me to do this. I know you're supposed to be able to do this because I've done it on previous playthroughs.
I thought there may be a bug with this particular gate, so I went back down to the Cathedral then made my way into the Blackreach and over to the Great Lift at Alfthand, and here it's the same - no action on the gate, so I cannot open it in order to come back down later on.
I don't fancy trekking through Alfthand every time I want to get to the Blackreach from now on.