Just thinking, I had Nominet open in Firefox, but to get Authy, I had to download Chrome browser and get that app on there, would trying to login on Firefox when the Authy app is giving me the 6 digit number on Chrome make a difference, I was assuming no?
[edit]
I think I've logged in 2 or 3 times previously using 2FA since installing it on May 8th/9th, I think all those times where on Firefox with Authy open on Chrome, in those instances, it wouldn't accept the 6 digit number the first couple if times, kept saying it was wrong, then let me in at the 3rd attempt, so it seems a bit pot luck whether it lets you in or not,