I found that "test broadband performance" and consequent test very misleading at first @Longshanks
It also automatically open "fault report" on MyBT, but absolutely nothing happens afterwards, up to point as within 10 to 14 days it's just disappear and that's it. no any subsequent call back, or proactive to try solve problem from BT.
Plus, small text will be there something as these test might be impacted by your local setup, traffic at a moment etc.
Glad, You was able to solve problem.
Quite surprised, while any cable over CAT5, CAT5E (both BT supplied are 5E), or CAT6 must be able to handle 150/49 without any issue.