It turned out to be a 10mm hex on mine. I only have a large Allen key in this size, but with a bit of a push I got the plug loose. The oil was very dark, so obviously it was worth changing it.
This flexible long neck funnel has been a Godsend for both oil changes, but particularly the gearbox. I had to remove the battery and it's tray to get proper access. The funnel was £1 from B&M, one of the best pounds I've spent on tools I think.