I know it's not fibre but when my parents switched from the up to 8 Mbps ADSL product to ADSL2+ which was syncing at around 20 Mbps with Plusnet, they were getting around 6.2 Mbps because the old ADSL profile had been left in place.

As suggested, you need to go to here (you will need to log in) and check Current line speed. That might be what is limiting you, if so, ask Plusnet to fix it.