Thanks fro your reply. An SSL cert is in place, and it looks like the cert is OK, unfortunately I think IPN needs sorting out because the site admin got an email from PayPal saying that IPN via HTTPS was an issue that needs to be corrected by 30/9/16
Extract from email from PayPal
Step 2: Understand how these changes affect your integration. Here are the key areas requiring your attention.
If the chart shows “Yes”, you may require changes to be compatible with that security upgrade.
If you see a “No,” our data shows that you are already compliant or do not use that functionality.
There may be other changes you need to make, but please pay particular attention to the following areas:
Do I need to make a change?
SSL Certificate Upgrade to SHA-256
TLS 1.2 and HTTP/1.1 Upgrade
IPN Verification Postback to HTTPS
IP Address Update for PayPal Secure FTP Servers
Merchant API Certificate Credential Upgrade
Discontinue Use of GET Method for Classic NVP/SOAP APIs