If the sync is showing 40Mbps then you have always had a problem. Which Plusnet product are you on?
If the sync was 17Mbps prior to the engineer coming then that would give an IP Profile of ~16.45 Mbps and a Plusnet Current Line speed of ~16.44Mbps. Although the IP Profile should rise immediately the sync does, the Plusnet copy (Current line speed) only updates from that a few times a day. That will be restricting the speed.
If you run a
BT Wholesale Performance Test, (ignore the red instructions and just say you've done them), then on the results page ignore those as well and go down to click
Further diagnostics. That asks for your phone number then gives much more information.
Please copy and paste the full contents of the two text boxes of results. We don't need the graphics.