Speed issues revisited

[#1]

As it will take a lot longer in moving than originally expected I have decided that it may be worth mentioning and investigating the poor speeds again with my line.
Latest BT Speedtest:
Your DSL connection rate: 4642 kbps(DOWN-STREAM),  1027 kbps(UP-STREAM)
    IP profile for your line is - 4000 kbps
    Actual IP throughput achieved during the test was - 2277 kbps
and my Stable rate is 4000.
Current specs are:
Sync   4643 DL   1027 UP
Att        45dB        13dB
NM        6dB          5dB
I have tried several speedtesters (mybroadbandspeed.co.uk, speedtest.net, speedtestbbmax.co.uk, numion.com) and they all give a similar figure of around 2400/2500 which is reflected in realtime downloads.
My question is simple really; should I just accept that this is the best I am going to get or can something be done to improve the situation?
Thanks.
Edit: Just changed the DSL mode on my NetGear DG384G V4.01.40 from Multimode to ADSL2+ and lost connectivity (downloads and browsing stopped, though no change in sync.). Changing back to Multimode corrected things - is this significant?

[#2]

Are you seeing the same kind of speed all the time? Or is it faster early morning/late night for example?
Also what speed does this say? (I'm at home and can't check unfortunately)
https://portal.plus.net/my.html?action=data_transfer_speed
If it's normal speed for some of the time then could be exchange side contention, if that page comes in at say 2500kbps then it's an easy fix. If that's right and the speeds are the same all the time then it's probably worth us raising it as a fault to see what happens, but as it's over 2Mbps it'll be hard to get anything done.

[#3]

Thanks for the reply Dave.
From your link the Current line speed is 4000.
The speed changes throughout the day. Sometimes throughput it can be low as 20KB/s even during the quietest times of the day yet in the evening I can see speeds of up to 380ishKB/s.
My concern is that, with the above stats, I wonder why I never sync higher than I do. I did notice last week that I had synced for a while at 5696 but can't remember exactly what the ATt and NM were - I think att was still 45.