After quite a lengthy wait from paypal's outsource tech support team there is a solution to this - it's definitely not pretty but here it is. We'll assume you've already been into the developer area and created a business account and a buyer account and that you've copied the sandbox api credentials into your application.
MOST IMPORTANT: CLEAR YOUR BROWSER'S CACHE
1) Log into your developer account area via: http://developer.paypal.com
Log on as your primary developer account - this now means your actual paypal account
2) You can then open another tab and log onto the sandbox site at: https://paypalmanager.sandbox.paypal.com/apiAccess.do?subaction=api
Log on as your business account
3) Open "another" tab (groan) and run your website and go through the payflow process and you'll then get to the logon screen where you can then use a buyer account to log in and buy things as.
Here's what's wrong with this:
You have to log on as the primary developer account first into developer.paypal.com, this, after they merged everything together is probably your main live buying account for a lot of people.
This means that you can't give test credentials out to clients very easily like you used to be able to, you could create a business and buyer account and give those details to the client to test their site, now it means you're forced to do a join.me or other shared screen session to show them it working.
I've raised this issue with PayPal MTS 2 weeks ago and had no response and it's marked as unresolved still.
I think if enough people lodge this as a complaint they'll tackle it. You can't log onto the sandbox without being logged in as the primary developer and the business accounts are linked to the primary developer account.