I've now got a little further.

The reason it didn't send out an email after commpletion was that the workflow was set to give it a status of 'Payment Received' once payment had been made, whereas the order needed to be 'Completed' before sending out info.

However, it still doesn't auto return from Paypal and it also still doesn't give the new user the role I've assigned for them once they've purchased.....

Tim