I think we managed to get to the bottom of the problem. Basically our CDN network changed some of the ip addresses they use, but they didn't tell us, so those new ips were getting rejected by the firewall we set up after the DDOS attacks. Now we've updated the ips that are allowed and so it should be fine again. Looks like they just changed it a few days ago, and they have lots of ips, so would explain why some members have had problems whilst others haven't.
Thanks again everyone for reporting the issue, and do let me know if you have any further issues. I'm hoping not!