OK, your problem is not line length because your actual sync is enough above the artificial 40Mb limit that line length won't make any further difference at this point.
It also is very unlikely to be a BT Openreach issue, again because the sync is above the 40Mb limit.
That leaves it either being to do with the PlusNet IP Profile which you said is set at 40Mb - can you confirm it actually states that in the "Line speed" link that several other posters provided earlier in the thread?
Other than that it is something likely within PlusNet's control. Could be you are on a "hot gateway" but often a disconnect and reconnect would jump to a different gateway. Or it could be some contention kicking in.
As far as the 38/40 argument whilst PlusNet were using the 80/20 product rate limited to 40/20 they usually (and currently) advertise it as 38 because they can only advertise figures that at least a certain percentage of customers can achieve.
EDIT : PS - the speedtest you posted
here is pretty much fine. I believe BT OR are probably confusing things by suggesting there may be something wrong with your line as it appears to be performing just fine from that result.
Edited by ian72 (Wed 06-Jul-16 15:44:58)