So the main questions:
1) What is your current ISP? current connection you are on with the ISP? i.e. what service do you have?
2) What exchange are you connected to?
http://www.samknows.com/broadband/exchange_search - put your post code in here. For example I am on the Wickham Exchange
http://www.samknows.com/broadband/exchange/SDWCKHM
3) Has it always been this slow? what speed does it report if you do a couple of speed tests on
http://www.speedtest.net/ - I get 39ms 1.76 Mbps down 0.46 Mbps up
4) Check your router connection stats, should have some stats on your router that display the bandwidth on your connection and the SNR margins etc, there are a lot of things in the list normally:
Bandwidth Down/Up(kbps) 2080 / 544
SNR Margin Down/Up(dB) 13.0 / 10.0
Attenuation Down/Up(dB) 63.0 / 31.5
5) Have you tried the test socket? on your master socket if you've got a standard NT5 master socket you should be able to unscrew the bottom half and then plug your filter/router straight into the test socket. The test socket will give you the best attainable speeds/performance of all the sockets into the house. The test socket is also the only one the ISP will support.
http://www.skyuser.co.uk/forum/view-master.html
Please note: after plugging into the test socket you may not get an immediate real world difference but the router stats may show your connected at a higher bandwidth.
ADSL works by updating your line profile, so if you managed to get connected at a higher speed for a few days it should update automatically. I had to raise a call with Talk Talk to get them to reset mine though as it wasn't taking the updated speed automatically.
I can tell you now for nothing if I hadn't tried the test socket I would still be stuck on 0.5 Mbps ADSL instead of having 2.0 Mbps connectivity, so it's well worth checking. If router stats still display the same bad speeds even when connected to the test socket you may have other problems with the line that aren't extension socket related, in which case an ISP switch may help (esp if you're going from ADSL to Cable or vice versa). However if you are switching from one ADSL provider to another and the line itself is the problem I can't see it helping much.
Your max attainable speed will be to do with the length of your line to the exchange itself, I have a long line here so I can't get any better speeds even on LLU.
