Background
I need to be able to test payments made via public paypal.com using a "dummy" personal paypal.com account created via the PayPal Sandbox tools: https://developer.paypal.com/webapps/developer/applications/accounts - here I created a Business Merchant Account (where the payment will be sent to) and also a Personal account (which I want to use to make the payment on the usual paypal.com public site.
The Problem
The problem is that the email address and password that I set up the sandbox personal account do not work on the public paypal.com site as I would expect - paypal.com reports the standard login failure. It also seems that the email address is not recognised - when I click the forgot password link and enter the same email, it says email not found. Surprising given that this email was what I used to set up the personal paypal account via the sandbox.
The question, summing up
So What would I need to do to be able to use a PayPal-sandbox-created personal account to test payments made via the public paypal.com site?
My Research on this problem so far
I've looked around already for existing answers - on SO and via google...
PayPal accounts, testing and sandbox (this doesn't help me - I tried the accepted answer about using a stronger password with special character but no difference)
how to login https://www.sandbox.paypal.com/ in personal test account ( the answer here doesn't help me as it's just a stock answer of instructions - doesn't actual help the asker - and the asker's problem isn't quite the same as my problem )