I don't think you want to use a RedirectMatch 301 type redirect if you aren't redirecting to the specific file in a different folder.
That type of redirect is used specifically for renaming folders. Which means that if you link to a file in the old folder, it will go to the file in the new folder. If you want to just redirect to a single file regardless of the file requested then you would use a standard RewriteRule you can still do a 301 redirect:
RewriteRule ^affiliates(/?.*) enquiries-booking-form.php$1 [R=301,L]