Thanks for that, that got me a little further!
In the latest RouterOS the url is actually: http://IP/user/signup/login
After doing that I still couldn't get it working...after banging my head against the wall I realised it's because user manager is running locally on the box.
I added a second IP address to the interface and rather than configure the radius with a loop back address (127.0.0.1) I gave it its own IP address and added that internal address to the walled garden. Hey Presto I can now access the signup page.
Mikrotik need to update their notes!!!