I have managed to get it to work. In case it's useful I'll detail the steps I took:
Note: I booked online through WDTC UK. Everything I did online was direct from the UK site - no forcing other international versions.
- I had a 6 digit confirmation number, which I knew would not work for dining.
- I called the UK number, 0800 16 90 730, and asked for the correct number to use to link my reservation with the dining system. I was asked a couple of questions about my booking, then was given a 3-letter, 5-digit code, and was assured this was what I needed. (This was actually the second time I called as the first time the CM told me there were system problems and couldn't find the code - so if you're not successful, keep calling)
- I was unable to use this code to access my reservations from the
My Reservations page, but, as it turns out, this didn't matter.
- On the
Disney Dining page there is a link
Use my dining plan to book pre-paid reservations. You need a Disneyworld.com account. If you are aren't already signed in you are taken to the sign in/create account page. Once signed in you are taken to the next step.
- You should have arrived at a page that says "Link Your Walt Disney World Reservation"
Then it asks for Confirmation Number, Last Name and Arrival Date. It won't let you enter an arrival date beyond 180 days, so you can't check this before then.
- Hopefully you will get a message saying the dining plan is linked, and will be able to book ADRs online out to 10 days beyond your arrival date.
Clearly I can't guarantee this will work for everyone, but my 180 day window opened a couple of days ago, and using my 3-5 code and the above steps I was successful.