i'm on the BT package,
if its the basic like ours the limit is 10gb, and you get charged £1 for every Gb over that limit, the first month you do it they won't charge you second month in a row you do it, they then tell you they'll chage you for every Gb over.
i was 1 gb over last month and i'll probably be the same again this month. so for £1 i don't mind. i blame xbox live
as for iplayer they eat through your downlad limit rapid. something like 1/4 to half a gb for each download.