80/20 speeds worse than 40/10 ... why should this be?

[#1]

I have newly joined PluNet and had issues from day one sadly 
I signed up online to the Fibre Plus service, but after speaking to the Openreach engineer he said I was only on a the 40MBs profile at the cabinet. Hey ho I thought not a problem! I did a quick speed test and was getting a quite respectable 28MBs (I am probably 700 yards or so from the Cab) and 8MBs Up. I then called PN who appologised and said the profile would be changed to the correct one and I should see the improvement immediately. After a week and no change in the slightest I rang them again to be told I was still on the 40/10 profile, was put through to support and it was changed there and then. Not a problem I hadn't been without service.
Since being changed onto the 80/20 profile speeds have dropped significantly, ping speeds have doubled.
I again contacted support, the issue was looked at and eventually an Openreach engineer dispatched to my home.
The engineer has just left, after telling me that my line and DSL box are testing perfectly.
Ok so why do the speeds decrease when the distances quoted before significant degredation are all quoted in Kilometres, whilst I and only hundreds of Metres from the cabinet?
This service is supposed to be the BT Infinity 2 which my next door neighbour is on and he acheives speeds in excess of 39MBs?
I spoke to support again who tested the line and quite happily informed me that my line test at a max of 23MBs! So I lose 12MBs on the 40/10 profile and I lose near enough 60MBs on the 80/10 profile is there a serious flaw in my maths or the technology?
Where do I go next? Why should the house next door, built at the same time, cabled to the same cabinet on supposedly the same service offering be able to acheive speeds twice as fast as me?
Hopefully someone can assist (*looks for the fingers-crossed emoticon)

[#2]

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Where do I go next?

Our faults team should be responding with regards to that ASAP. It may be the case that another engineer visit could be the way forward.

[#3]

Thanks Adam, another visit or I'm happy to assist in getting something sorted any way I can.

[#4]

I can answer a few of your questions, so you at least understand where you stand. Whether it helps fix anything is another matter...

Quote from: landylover

Ok so why do the speeds decrease when the distances quoted before significant degredation are all quoted in Kilometres, whilst I and only hundreds of Metres from the cabinet?

FTTC almost certainly degrades to zero in less than 2km, probably around 1600m, with the upstream disappearing first. It would probably stop being considered "superfast" between 1200m and 1500m. Ideal distances to get top speeds are within 500m.
My line is 400m, and is pretty clean - but only just supports 80/20 (while the speed check predicts 55/18 for it). I had an previous line of 650 metres that behaved rather differently. I'll explain that later...
Those distances relate to lines made of copper of decent specification, with decent connections. Some lines (especially for houses built in the 60's and 70's) are made of aluminium - and speed degrades faster with distance on that kind of cable.
What does the BT Wholesale checker predict for your line speed? I'd imagine that, for 700m of copper that the prediction would be 35/10 or so. Less if the cable is aluminium.
And is your 700 metres an "as the crow flies" distance, or following the roads, and including the extra distance between the 2 cabinets?

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This service is supposed to be the BT Infinity 2 which my next door neighbour is on and he acheives speeds in excess of 39MBs? Why should the house next door, built at the same time, cabled to the same cabinet on supposedly the same service offering be able to acheive speeds twice as fast as me?

It is feasible your line takes a different route, or goes to a different cabinet. Perhaps not likely, but it is certainly possible.
However, it is possible that your line gets more interference than their line - and this interference is causing more errors on your line than theirs. The interference might be caused by a fault on the line (and is therefore fixable, eventually), but it might be caused by other FTTC customers (and isn't fixable until new technology arrives).
If, for whatever reason, you are getting errors on the line, then the cabinet can adjust your profile to either add interleaving, or to reduce speed, or both. The process doing this is known as DLM - and it tends to trigger changes around 4AM-8AM, causing a resync at that time. It is quick to downgrade a profile after errors (within 24 hours) but slow to recover once the errors go (up to a month). In using this process, BT are (very much) favouring a stable line over a fast line.
Adding interleaving is noticeable, in that it adds latency to ping times - so this certainly looks to have happened on your line. A side-effect of turning interleaving on is that it also turns on an error-correction process. If the errors are bad enough, then interleaving (and error correction) can be turned up high - but this can use more than 20% of your available speed so that error-correction can work.
What *is* strange is that the change of package from 40/10 to 80/20 should drastically lose you speed. At 700 metres, you were probably never going to really gain much, if anything, but you shouldn't (theoretically) see such a drop. Yet it seems to be happening to a fair number of people, and I haven't yet heard an explanation (from *anyone*, not just Plusnet)..
Now my previous line of 650metres... It got errors from day 1 on the 40/10 package, and you could see the effect of DLM shifting profiles: latency was added to the ping times, and speed dropped around around 3Mbps because of the overhead of error-correction. That line, at first, was only just capable of handling a full 40/10 speed even if it didn't see any errors.
Later on, once BT re-configured the cabinets to be capable of 80/20, but while I still had a 40/10 package, the line became capable of a speed a bit higher than 40/10, and DLM turned my line back up to a full 40/10 speed. In that case, the errors didn't disappear, but the extra (small) hike in capacity let me get a full 40 while the error-correction overhead was taken from some of the new capacity. Unfortunately, we didn't keep that line long enough to see what happened with a full 80/20 package - but the changes clearly shows the impact of interference.
What does this mean for you?
I suspect it means you are getting errors on the line, and that the cabinet has shifted your profile to cope. This explains the hike in ping times and at least some of the slowdown. The profile change may have slowed you further as an extra precaution.
If that is true, then the job for the engineer is to determine whether you have some kind of a fault on the line (eg a dicky connection in a manhole, or up a telegraph pole, or in a cabinet), or whether you are merely encountering "expected" levels of interference.

[#5]

Hi landylover,
I can see that your fault has been updated and been assigned back to our suppliers for further investigation. We should be receiving an update very shortly, once we have this our fault's team will be in-touch.
Please let me know if there is anything else I can do.
Chris