I'm making good progress now, and have done quite a bit more of the knot.
It has a definite start-finish point as the horse's tail, so I started there and followed the line:
It falls into four quarters; the tail starts half way through one of them, but after that I could do a full one:
It's really starting to look like it ought now, isn't it.
The plain knot is quicker to do than the inner loop (no fiddly leaves), so hopefully I should be able to get on with the rest pretty quickly.