The problem is that when I go to a speedtest site like speedtest.net I'm told my connection is 30 down 10 up, as I'd expect since downloads are limited by the firewall. If I try to actually download anything though the most I ever get is 2Mb (about 200KBps as reported by the browser).
Anyway I immediately thought "traffic shaping" and called the ISP to shout at them, they however deny that they do anything like this.
Cutting a long story short I've tried another router which behaves in exactly the same way and I've also tried the speedtest on this site: http://www.thinkbroadband.com/
Given those results that site suggests:
The speed test looks at various parameters, and in this case there was a large difference between the two download tests. A common cause for this on older computers (e.g. Win XP) is that the RWIN value is too small, connection tweaking software can improve this. Another reason can be congestion, either with the broadband provider or others in the home using the connection.
If the issue is congestion the two results will generally be better at off-peak times, so try running a test after midnight and before 3pm in the afternoon.
I can't honestly believe the rwin setting can have anything to do with it, none of the equipment I'm using is that old and some of it has been used on similar connections from the same ISP.
No one else is using the connection when I'm testing it.
I've tested at 1AM and it's no different.