Thanks for all the replies, guys.
Unfortunately, I don't get to fiddle with most of the settings, as it's managed by our broadband provider.
Here's what I've found out:
1. Firmware version is v4.0,build0656,130211 (MR3 Patch 12)
2. If I disable AV it goes to 32Mb down
3. As for load, we've got 2 servers, about 60 workstations, 25 laptops and a bank of 30 netbooks. Unifi managed wifi too
Thanks
EDIT:
Here's the reply I got from them: