OK; two problems, both of which should
automatically fix themselves.
Your current speeds are most likely due to the line issues you did have.
BT's "systems" will select an 'IP profile' which closely matches the lowest connection speed you've recently suffered.
If you do a speedtest using http://speedtester.bt.com/
the result will come back with your current IP profile (I suspect it'll be 135kbps).
Now your speed has recovered the "system" should respond to this and bring your profile back up (probably to around 3000kbps).
Unfortunately this can take up to several days (maybe five at most).
For large steps; it is supposed to happen almost immediately, although sometimes this doesn't happen.
be able to ask BT Wholesale to reset the IP profile on the line and BT should
do this (in italics because many ISP staff won't understand what you're asking, and BT are notoriously awkward).
As Matt has said, your 'target margin' has now gone up to 15dB (due to the line instability you had).
Unfortunately (and even though this was due to a line fault) there seems to be no mechanism (at BT) to automatically reset the target (back to the default 6dB) when BT fix line faults. They prefer to rely on their automated "systems".
The automated "system" (DLM) should
, over time, drop the margin back down (bringing your speed back to its original 5Mbps).
Unfortunately; it often doesn't (I've seen plenty of perfectly good lines, one of which now connects at 12Mbps with O2, where BT's DLM had decided the line was so poor only a fixed 15dB margin was suitable) - basically it gets stuck.
Even when it does come down, it does so in 3dB steps, and seems to take forever.
Once it's back down (if it ever gets there) it seems to remain fidgety, and often goes back up at the first sign of a router re-sync/reboot.
Both of these problems can be resolved by your ISP.
They can ask for the IP profile to be reset and for the target margin to be set back to 6dB.
The problem appears to be many ISP staff are poorly trained (especially the ones on 1st line phone support - who just rely on scripts written by someone else) and if you don't word your request in precisely the right way, they get all confused and keep saying things like "you get what you get, if you're getting 135kbps, that's all your line will do".
The second problem seems to be the way the ISP reports the fault to BT. Either BT have equally poorly trained staff or just refuse to do stuff if exactly the right fault code or request isn't used.
I got so fed up with BT's stupid IP profiles on my line, I left the BT system entirely and went to BE unlimited (as they don't have the IP profile system - your speed is around 85% of the connection speed, no matter what the recent history is. They also don't have DLM which, on BT, mucks about with the target margin - affecting connection speed - and is notoriously over-sensitive).
Hopefully one of the Plusnet staff will see this post and know exactly what to do.
Other ISPs (particularly AAISP) could fix your problem in a very short time (and migrating away from Plusnet may be your only option if Plusnet can't/won't fix the problem).
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware