Disappointing speed after uprade to ADSL2+

[#1]

My line has just been upgraded to ADSL2+. Previously I was able to get a download speed of over 6Mb on the 8Mb service. This has improved since the upgrade, but the best I achieved has been only 12Mb. I expected better than this as the service is up to 20Mb and the speed I was quoted when requesting the upgrade was 17.5Mb. My router is in the same place as it has always been. Are these results typical or should I expect a faster connection?

[#2]

All quoted speeds are estimates and no guarantee of what you will actually get... It is totally dependent on your line quality.
What are your modem line stats? What does a BT speedtest give you? What is your plusnet IP profile set to?
Are you using a wired or wireless connection when you did the speed tests?

[#3]

I am using a wireless connection (the router is on the 1st floor, my computer on 2nd floor), but my wireless card informs me it is connected to the router at 36Mbps.
My router indicates connection speed of 15585 kbps, attenuation of 20db and noise margin 6db.
The BT speed test indicates a connection rate of 15585 kbps, with an IP profile of 13000 kbps.

[#4]

Wireless connections can cause slowdowns, especially when you are not getting the max speed (whch I assume would be 54Mbs 11g in your case). You can easily only reach a 3rd of the connection speed on wireless due to encryption and protocol overheads.
Can you try connecting  wired and run the speedtest to see what the real speed is. If that is as expected then you know the wireless connection is your issue.

[#5]

A touch low sync for the attenuation.

[#6]

Peter,
I appreciate that using a wieless connection makes a difference, but surely the fact that the BT test gives a connection rate of 15585 kbps, with an IP profile of 13000 kbps shows that the connection between the router and Plusnet is the restriction.
Glyn.

[#7]

When i was on a 13MB profile i only got 12.2mbps
not sure why its pretty fail.

[#8]

Any connection rate between 14700 and 15900 gets allocated an IP Profile of 13000 by the BT DLM equipment, so defining the maximum throughput. Plusnet get told the value so they can restrict the speed on their side to match.
With that IP Profile value, Speedtesters will generally show download speeds of around 12.4Mbps.
The difference between the 13000 and the 14700 is because your data is encapsulated within packets that use up the extra as overhead. There's nothing you can do about that.
The gap from 14700 up to your sync of 15500 is truly lost, and down to the choices of BT in employing a profile system. There is nothing you can do about that with BT-based broadband suppliers.
So... So far, you seem to be getting a reasonable speed result for your sync rate/connection rate. As Epyon points out though, the connection rate seems a little low for the attenuation.
You could go to http://www.plus.net/support/broadband/bbfaults/slowspeed.shtml and click on the link to "Downstream rate is too low" for advice.