Thanks to everyone who replied. The info was helpful. The Avast dialog didn't say what the malware was. It sounded like the URL had been blacklisted previously. As you were convinced the site was clean I reported it to Avast as a false positive. They have replied since to say it was checked and its reputation has been cleared in their database. I can login to my normal account now without disabling any protection. It should be clear for all Avast users too.