Hi @Gareth_Furnell , have you solved your issue?
Did you have a mongodb account before you tried to sign up for the course?
The login/signup page might get stuck sometimes and show weird things on the page. But once your registration is complete and login finely, you will find it is easy to navigate around. (the browser you use may affect the result).
I suggest you first login to the Mongodb University Learn main page first, then try to navigate other courses, and if they seem alright navigate to the link you gave for SI course.
I am sure it will be fine after that point on.