Eugene's observations

The perennial problem of how to maximise the possible broadband speed continues to cross my inbox.

First some ground rules:

• You cannot change the most common significant limitation - the distance between your house and the exchange
• Changing to a different internet service provider will typically not affect the signal speed - you use the same wires leading to the same exchange as always

However, the following issues are within your control and do affect the speed you receive

1. Concurrent use by more than one computer, tablet, wireless phone etc. causes the speed each device receives at to be lowered. Even if not active, background tasks such as virus or Windows updates can be in progress
2. A faulty or obsolete router/hub that does not support the ADSL2+ standard will pain you
3. Faulty equipment connected to your circuit (microfilters, phones, faxes, satellite box, security, IP TVs, ringers, boosters?) can cause speed problems
4. Malware running on your computer is often at the root of the problem
5. Poor quality cabling within your house is a drain 
6. Siting the router away from the master socket increases distance to the exchange (yes, that small amount IS significant) and usually introduces wires with relatively high aluminium content that is fine for phones but allows too much crosstalk for a satisfactory DSL signal
7. Using wireless, especially the older 802.11g standard (at either end), rather than wired connection to the router will lower the speed - perhaps by half
8. Interference with a wireless signal from cordless phones, microwave ovens, white goods, masonry, and other equipment that shares the 2.4 GHz frequency is a common issue

The above list will often provide the necessary pointer to the resolution of your problem.  But not always - faults beyond your premises can occur.  In that case here is what needs to be done

Run several speed tests at different times of the day to get average figures.  The place to do this is 
http://speedtest.btwholesale.com/ after the initial test completes click the 'further diagnostics' button.   Now find out the speed that the BT Wholesale Availability Checker database thinks you should receive (this is only a best guess) from
https://www.btwholesale.com/includes/adsl/main.html

Compare the DSL downstream line rate with your line speed as reported by your ISP or available in some top end routers.  If there is a significant difference (>15%) and your line has been installed for more than 10 days then see if the phone line sounds crackly. If so, a phone fault is the cause of your broadband speed issues.  Otherwise, replace the microfilter (and ideally the router) at the master socket and see whether the problem is resolved.  Note that you should disable all internal equipment by connecting to the test socket behind the facia of the master socket.   If the problem is resolved then internal house wiring and/or the old microfilter/router are at fault.

Your IP Profile speed should be just under the DSL line speed.  The latter will always be higher by a small amount as housekeeping accounts for some speed loss.  If there is much of a difference your ISP can be asked to reset the profile at the exchange. This causes an automated 10 day sequence where the fastest speed your line is capable of receiving whilst maintaining a stable connection is determined.

If the above two items check out but your download speed is low then either one of the eight items above are at fault or there is a fault at the exchange equipment.