So, I come back today with a story of failure. Paid for my ticket online last night, well beyond the "allow 2 hours", and when I went to to the station, "Error 57, Seek Assistance", from every gate I tried. The TVMs also just state "Invalid card".
Spoke to the TO - they can see the ticket on my online account, but can't see my card being validated at the gate line. They haven't got the ability to interrogate the card directly like LU were able to with Oyster cards, but they do seem able to put the card number in manually and view the details that way.
Then spoke to SE Customer Service - apparently, others are also having problems and there are known issues collecting tickets at station gate lines. If you had your ticket pre-loaded when initially ordering the SE Key, you're fine, but it seems that if you bought a ticket online for collection at the gate, this is the bit that isn't working.
I've Tweeted SE several times for information about this, and they've given me the silent treatment so far, which is unlike them in my experience. In the meantime, I've just been advised to keep trying, but to also speak to gateline staff to be waved through manually until the problem is resolved.