I just got off the phone with the DVSA and the person said that he'd need to contact the DVLA to check that I was eligible for the test as I have a Danish EU license. If you call DVSA on 0300 200 1122 and have your Driver number from your D9 form, then hopefully you'll get the same help as I just did.
I haven't actually booked the test yet, but at least it seems like someone is on it for me.
Edit to Add:
Just got of the phone again and he had to book it in manually for me, but I now have an appointment for the test. I assume it's something to do with the EU license, so definitely call them and ask about it.