Finally managed to get the upgrade to Fibre 900. They either fixed the database issue or changed something at the exchange as the max available was 330 until I checked again yesterday. Gave them a call, told the best they could do was £67 per month. Called again, this time to cancel (was out of contract), was offered it for £55pm, which is a fiver cheaper than I was paying for the 300 package.
Upgrade went live overnight, wired speed tests seems to top out around the 500-600Mbps range. Upload speed is excellent though. I know technically you should get the max speed straight away but did anyone find their speeds improved over time?