Hi Mark,
Apologies for the delay in response to your email. I sent a response yesterday, copied below encase you've not got it yet. If any of these help, can you report back here encase other users also have the same issue.
What is your domain? Have you any server restrictions like ip blocking or the site behind a .htaccess user login?
Can you also ensure your PayPal account has the ipn tickbox enabled. You can leave the ipn url in PayPal blank.
Thanks,
Adam.