So who do I contact in BT about this (please don't tell me 150)
Use the Broadband troubleshooter https://www.bt.com/help/broadband/fix-a-problem/broadband-and-wi-fi/bt-broadband-connection-problems
Doesn't answer my question tho'
If it reports that there is an issue with your speed, then you book a visit from Openreach to investigate. Remember quoted speeds are connection and not download speeds.
As mentioned if you believe you have a fault it can be reported online an engineer booked if needed. If you would rather to speak to someone then speak to the technical support team who will run a test for you.