Well since the magnetmix download was slow, I'd be looking at your ISP as a suspect

No, seriously, since they have that FUP in place & since speeds have dropped off since you've been with Sky Broadband...it sounds suspicious.

Especially since you're not even seeing the firewall icon...being firewalled can slow down speeds. That's not your problem. If changing ports didn't help, there's not too much that we can do. If it's an ISP issue then there's probably not much that you can do either, apart from changing ISP