Why when I test my broadband connection using Now connection test in my account does it say 80mbps, but if I test it with any other broadband speed test I get between 5 and 13mbps.
Because that is your sync speed, see if this helps explain it:
and speed testers measure the real throughput. But where? If you are measuring this with a device some way from your router, you may just be measuring how your WiFi drops off as you move further from the router,
So try the test right up close to the router, or even better, over a device wired to the router with an Ethernet cable, where you should expect to get 80-90% of your sync speed.
But if you still don’t, get on the phone to Now Broadband and raise the issue.