Hi Mark,
Three things to try:
Try powering your router down and, after a minute or so, powering it back up again. Routers cache addresses and confusion abounds if they get corrupted as mine did during our last thunderstorm. A full reset causes the router to clear the cache and start again with a clean slate. As you have three different machines with the same fault, I think a router problem is the most likely suspect.
Disabling your virus checker won't help if web checking is being done by another software package, even if it is from the same vendor. You might have to look closely at your security set-up to find out what's blocking the site.
Related to point two, I tried to find out what the IP address of the site was to see if it could be called directly. The address is 212.48.75.22 but the website can't be accessed using it. I don't know if it's a security feature at their end or to do with the way the service is made available by their provider. I don't believe it's sinister, but the peculiarity may be upsetting your security software. If so try switching it off or using another product.
Good luck,
Dave