okay I have tested this issue for the last 6 hours and I believe it must be a setting in the test buyer account on Paypal sandbox. Come to this conclusion because Paypal live purchases do work.
However i did encounter 1 issue, the download image is not the correct size nor is it the original size . I selected in the options to have the download use the original size if it was not available.
1. Can you assist with what you understand are the best settings for a Test Buyer Account within Paypal Sandbox ?
2. How do I ensure the correct download image is linked?