Hello Steve. Thank you for the comments. Here is the additional information.
FULLER CONTEXT
I want members to only book services after they complete the Member Profile (where they must accept the T&Cs to continue).
When existing members login, the site sends them straight to the services page, where they can book.
It is different for new members who wish to sign up.
I have Signup buttons on the main body of the page, which are set up as “mode: signup”, so the redirection is as I need it to be. However, the Login/Signup button in the nav bar only works when completing a login. If a user switches to Signup mid-process it won’t do what I need it to do.
CURRENT URL
I hope this clarifies the need