If you have the BT home hub, connect that for 1 or 2 months, as that shows you your downloaded amount, I would do that just to make sure that BT aint coming wide with you, I did use netmeter to check how much I downloaded on one particular machine, but as kitz says, it only works for one machine so if you have a few pc's then it wont give you the exact total as you will need to run it on all machines then add the total of each.
best bet is to use the bt hub if you have one just for a month, just to see if your figures and bt's figures tally, if they do then go back to using your old router.
I have a brandnew bt home hub for sale if you want one
another thing you might like to know is that on BT's website, they state that they DONT charge for over usage nor do they cut you off and that they dont perform any traffic shaping/throttling in anyform.
ALL of them lies.
Well atleast with the residential broadband
since you are on option 4, you are probably better off migrating to BT business, even if you are still in contract with bt total, your contract gets transferred to bt business and you get unlimited downloads a month and a magic router for ?32, roughly what you are paying now, maybe a pound or 2 dearer, but you get UK support and a 8-24hr problem fix turn around.
Thats what I did, I signed on with Bt total and hated it for the 3 weeks that I was with them, I then migrated free of charge to BT business, and havent been happier, no download limit, no problems
and no, you dont need a business to have bt business broadband