It is unclear from your posting what the setup is.
"With our ADSL line it is easy. Simply go to speedtest.net and test"
Speedtest.net (whose results vary dramatically depending on the host selected), measures the speed of your broadband connection. It doesn't measure the wireless speed between your computer and router. This will have a nominal speed of 54Mbps (802.11g) or 150/300Mbps (802.11n).
Your posting suggests that the building gets its actual broadband wirelessly, so the antenna on the roof will be connecting to some sort of "receiver/router" to which you will connect by ethernet cable. If the "receiver/router" doesn't allocate addresses automatically by DHCP, you will need to enter these manually in the Local Area Connection's TCP/IPv4 settings.
- IP address
- Subnet Mask address
- Default Gateway address
- DNS Server address
Then, when you have got online, perform your speed test.