ipconfig /release is only going to work on the internal network, not on the internet.

Virgin are correct that it will change (or CAN) change every time you restart your router, whether it does or not is just dependant on their DNS timeout.
Anyway, there is no way to change it to a specific, or old IP address, but if you switch if your router for a few days it is likely to receive a new one.

But the problem you are having doesn't sound like its attributed to this anyway, whats the site and whats the exact error you are getting?