An ECI headend could explain the 330Mb max download , but could not explain why other ISPs that are using the same headend can offer 900Mb , after all the CBT that serves your property is only optically connected to one OLT ( headend ) via a splitter , so optically they are identical to BT .
Its possible that the ‘cable links’ that connect the OLT into the various ISP ‘backhaul’ equipment are the limiting factor , that would explain why some ISP that don’t use the BTw backhaul can seemingly offer 900Mb, when BT cannot ( obviously BT as an ISP , won’t use the same backhaul as Talk Talk or Vodafone for example ) or it could conceivably be a legacy of replacement equipment ( ECI swapped out for a manufacturer that does provide 900Mb ( soon to be 1.8GB) ) but the internal BT records have not been updated to reflect this where other ISP’s have.
The problem really is , because you can only speak to the ISP , when the issue is really an Openreach/ BT Wholesale one ( or whatever BTw is currently called ) you will only ever get someone in BT interpretation of the issue , as you are not allowed ( by the regulator ) to speak directly to BT’s supplier .
Have you considered emailing chief executive office , the executive complaints resolution people may be able to delve deeper into this , whereas a regular customer service representative may come up against the regulatory barriers
