It turns out I filled it incorrectly (same day and month, 1 year out) on the divorce application. The STBex didn't notice when they filled out their response. The court also did not notice the discrepancy between the application and the copied marriage certificate!
The Decree Nisi has the incorrect date on it. As does all related paperwork. I've called the wikivorce helpline but wasn't able to get through (I've asked for a callback). I'm not working with a solicitor on this; it's not a complicated case (no dependents or shared assets) and it's been pretty amicable.
So. Anybody got any idea what the heck i need to do to untangle the mess that a single digit typo may cause me?! Or if it will even cause me issues?
Wikivorce helpline called me back. The lady I spoke said I need to try getting in touch with the court and asking for a correction to the Decree Nisi under the slip rule (not one I'd heard of previously!).
I made the call, and after some time on hold I got a very nice, apologetic court officer to agree to ask the judge for the amendment. Hopefully it'll go through without too much pain and I'll be back on track soon enough!