FTTC line running at third normal speed

[#1]

HI. For the past couple of days my internet speed has been a third of what is normally is. I normally get 44 mb download , 9 mb upload and 15-18 latency
http://www.speedtest.net/my-result/3181274147
I'm now getting 15 mb download, 9 mb upload and 20-25 latency   http://www.speedtest.net/my-result/3307682118
Everything looks fine on my gateway, the PlusNet site says 'Estimated line speed: There's no speed estimate currently held on your account. Current line speed: 46.4 Mb
But the BT site says
1. Best Effort Test:  -provides background information.
Download Speed
15.91 Mbps

0 Mbps 47.04 Mbps
Max Achievable Speed
Can anyone suggest what might be happening?
Many thanks

[#2]

Are you testing with wireless or wired ?
Sometimes a neighbour gets a new router on the same channel as the one you're using and it messes things up a bit speed wise
Also have you done a "quiet line test" on 17070 ? Any cracking or interference could cause your speeds to be reduced.

[#3]

@Cascy
Line tests aren't revealing anything obvious to me so let us know re what DomS has mentioned but I'd advise raising a fault here: http://faults.plus.net