If anyone wants a full walk through on the technology I am happy to post it.
Highlights.
DSL came in and gave up 2Mb depending on distance from exchange >3Km
DSL+ came in and gave 8Mb at the same cable length
BT Infinity is known as FTLOC or Fibre to the local cabinet gave 36 or 72Mb depending on quality of cable and distance from local cabinet. This means driving a faster uplink from the exchange into the national backbone and then instead of homing the copper links all the way back to the exchange you distribute it to local cabinets allowing a shorter/faster final copper link.
The van may have Infinity on the side but this has no direct bearing on what the engineer is fitting !
All of these are download speeds and the upload speed is usually a 20% max of this. None of the carriers (BT etc) want you running a datacentre in your house as this this would stop them selling you space in their datacentre !
Contention is a separate thing that applies regardless of the click speed on the line and shows how many people you are fighting with for the up link. As an example if the provision 100Mbits to a local exchange they will sell 20 times that and you can all fight for it at peak periods. This shows as a quick response but still a long time to download during peak periods as you share with more people. Home can be from 50:1 down to 20:1 and "Business DSL" packages are usually better to provide consistency as well as better support for faults.
Download volume called "unlimited" is the total amount downloaded but can still also be split by "peak times" or typically 5pm-8am which is when residential broadband gets heavily used. You may get unlimited but during peak periods if you go over an average they will rate limit the % of the volume you get. This is referred to as throttling or choking your connection to give others a chance. Check for fair usage policy in the Ts&Cs
In many cases you don't feel the extra speed on a single machine when you go from 8Mb to 36Mb but this is due to the problem being that the remote host you are linking to (web server pushing pages to your screen) simply is not fast enough to push them faster in line with your new shiny super cable. Where you will see the speed difference is if you use a TiVO or Sky+ to download a video in which case it will download very fast. You will also see it when many users in your house are surfing at the same time, response will not appear to sag under load.
When you are using speed test you need to consider a couple of things. For a speed test to be useful it needs to be done with a single PC cabled (100Mb at least) directly to the router with no one else connected. As broadband speed is a simple measure of the click speed from your house to the exchange (and so nothing to do with the dozens of onwards hops to the server and thousands of others you share these with) it should be tested in this manner.
The best way of doing this (if you are on a BT based link) is the wholesale broadband speed test site.
http://speedtest.btwholesale.com/ in general this will test the local loop from your house to the local fibre cabinet or exchange.
Using WiFi to test performance is bad as this is a very variable medium, if you are on the Infinity 36 or 72Mb service even 802.11n will have problems giving you a clear run at an honest speed test. If you are on 2/8 Mb DSL then a good signal should be less of an issue if you are in the same room.
If you are testing against a remote host then the hop from your PC to the router, to the exchange/cabinet, to the core network point, to the backbone, to the remote datacentre, to the host is all in the chain. As is the performance of the host at that moment. It represents that path to that host at that moment. The web is not that simple to speed check.
If you are on Virgin/Cable all of this is not directly useful. Cable is more like FTLOC due to its architecture. It should be comparable to the Infinity for raw speed but it can suffer from a lot higher contention levels and so slow during peak hours.
If anyone wants any help they are welcome to PM.
