Ok – ive edited this as it seems Flickr have changed their ways again. Luckily I just found an option in the site to let the site do all the hard work.
When in Flickr you can just grab the url at the top of your browser in the address bar and then paste directly into where you are typing – do not use the img button above!
Try again – still didn’t work, my attempt came out the same as yours Offcuts, and I selected Medium 500 too. In fact, it seems larger than medium 640 – maybe you automatically get the original size? Might be worth raising a query with Joe.