Thanks for the reply, Falko.
Yes, the Traffic Shaping Administration page allows for prioritizing according to traffic type, but it doesn't prevent an individual connected to the net via the corporate or home network from downloading truckloads of content in the times that there is no high priority traffic.
My goal is to migrate my systems to Linux and GNU wares but the only option I can see at the moment is to maintain an MS ISA 2004 Server with the 'Bandwidth Splitter' addin (http://www.bsplitter.com)
This enables the monthly quota to be divided amongst subnets, users, or clients - to be used however they wish.
I am hoping there is someone experienced in Linux who can advise me if there is a GNU solution for this component of the network infrastructure widely available at the moment.