See this link for "Pay my bill" and also the number to call if that fails.
NOW Broadband Members can call us 8am–8pm, 7 days a week.
The link to the page is what I have tried over and over. I can't ring as don't have credit on phone. Using works WiFi atm. This happens last month aswell. I can't remember how I eventually got it to work but it wasn't by ringin