1.5 mb commection is way to slow for option 3

[#1]

@exasperated
Firstly I'd better point out what many here know, I'm not on PlusNet's broadband, though I have been.
Your problems are difficult and of course "exasperating" but I can say that of all the ISP's I've used (and it's a fair few) PlusNet are the most likely to work with you to get to the bottom of it.
Please bear in mind that this could be something (e.g. external interference) that is totally outside PN's, BT's or your control. Finding it and dealing with it may be difficult or actually impossible. Comments such as you report a BT engineer saying "my ISP is throtteling the line or words to that effect" are neither helpful nor accurate, more like a lazy or insufficiently trained engineer "ducking the issue".
And please note the words "work with you". If it was easy they'd have done it by now, so please treat them with the same respect as you expect them to treat you.

[#2]

iv read this post from start to end. and like every one else that has replyed i surrgest the same find the cause of the line fault as it follow u to any isp that uses bt's network.
just a couple things i would try is
1. try a diffrent router ( as with bt man pluging his lappy in to your line and getting full sync would make me think its a problem at your end. but that said bt engeniers dnt seem to like to work unless they have a supply of tea an biscuits so it may of bein a lie just to get home early)
2. u say u have done a quite test on your phone. if with every thing unplugged and doing test on a standard coreded phone you have noise on your line you need to raise a voice fault with bt and do not metion broadband to bt.
good luck sorting this problem out if u have allready requested your mac code as u said u would on sunday. As this problem will follow unless you go to cable and virgin media are one isp i would avoid at all costs. or if the problem was at the exchange may b moving to an llu isp a change of ecupment at exchange may sort it but unlikely.
one thing that coould happen if u move to a resller of bt's network they may send u new router with the package and u find u get your sync speed back and you will think it was plusnets fault. so please try a diffrent router b4 moving on.

[#3]

two words
"line fault"
not PN. I have learned the hard way about noise on the line, even small amount completely squishes your speed.
Turned out i had a lose wire in one socket in the house. If you have noise on the line on a silent test is got to be a line fault, ether with the wiring in the house or outside, and NOT PN.
ps: I work in tech support for another firm, i dont like bananas and i sure as *beep* dont like like a moneky.... just maybe a very round pot bellied pig 

[#4]

I wish you luck Exasperated.  I live in the middle of London and I used to get 2mb /sec and this has dropped to 400 kb now.  Plusnet cant even get BT to test the line here and PN failing to read all the ticket drives me mad:
Test1 comprises of Best Effort Test:  -provides background information.
    Your DSL connection rate: 800 kbps(DOWN-STREAM),  448 kbps(UP-STREAM)
    IP profile for your line is - 500 kbps
    Actual IP throughput achieved during the test was - 448 kbps
If you wish to discuss these results please contact your ISP.
BT line services say my speed is not acceptable but it is up to PN to raise the proper failt with them - they asked them to test the exchange which they did, then closed the fault!
BT also said that if moved to them,  they have more clout with Openreach.  so now I have my MAC key from PN I am tempted to move away from PN.
Good luck and enjoy 1.5 mb while you have it,  with plusnet's help it might turn into 400kb!

[#5]

Quote

BT line services say my speed is not acceptable but it is up to PN to raise the proper failt with them - they asked them to test the exchange which they did, then closed the fault!

I'd be interested to see if that was the case, as they shouldn't, especially as PN should have exactly the same relationship being a BT company.

[#6]

Quote

Now its 1412, better than the last few days but at 05.30AM you'd think it should be faster.

The ISP has absolutely no control over the speed at which you sync to the exchange at.  Thats between your modem and BT's DSLAM
The fact that you said when an engineer arrived and he was able to sync at 5600, would point to the fact that the fault is either with:-
~ Your equipment or
~ Your internal wiring or
~ You have a line fault which wasnt present for the short period of time that the engineer was there.
The BT engineer has given you misinformation, this is not an ISP throttling thing..  and Im also curious as to how he says getting the isp to reduce the target SNR would make that kind of difference because he sure as hell will have had the same restriction when he used his test equipment to sync.
Ive had BTw try tell me some wonderful porkie pies over the years trying to pass the blame to the ISP.. but unfortunately for them they pick the wrong person to try it on

Moving to another (IPStream) ISP wont solve this issue and as mentioned Plusnet are one of the ISPs whose staff have least half an idea how adsl works and will attempt to get it fixed.
Believe me I come across many users with faults from all ISPs on a daily basis and there are some ISPs that make me bang my head on the desk. 

[#7]

Quote

BT line services say my speed is not acceptable but it is up to PN to raise the proper failt with them - they asked them to test the exchange which they did, then closed the fault!

Interesting!
By line services I pressume that means PSTN?  In which case how the heck did they have access to adsl information to raise an adsl fault?
Surely they should have said something like no PSTN fault found, therefore you need to raise the issue with your Internet Service Provider so that an adsl fault can be raised.