I now have Paypal working!
For the benefit of others I will show how I did it.
Contrary to many different sugestions the 2 paypay entries in a2billing.conf have to be:
paypal_payment_url = "https://secure.paypal.com/cgi-bin/webscr"
paypal_verify_url = "www.paypal.com"
and you need to login to your paypal account and turn on auto return and enter the return address of your payment processor. ie. (for me using elastix 1.6 and a2billing 1.3.0 it was
https://myip/a2customer/checkout_process.php)
The documentation on the a2billing site showed the paypal_verify_url="ssl:www.paypal.com"
which does not work for me (failing in the logs)
These are the ONLY changes I have made, hope it helps someone...