OK (well, not OK, but at least it's clear which ISP you use - in one way a great advantage, as you can {more} easily switch ISP if you decide to, later on).
Next question is whether there's only a single PC being used, and if so, just what types of things you use it for (if you use SeeSaw for TV shows) or 4oD / Demand Five / iPlayer (watching whole TV shows), then every half hour or so will clock up perhaps 250 MB of traffic...
...so... watch 20+ hours of decent (not YouTube) video, and you might be getting close to your monthly limit.
I only ask if there's just one, or more, PCs, because it's clearly easier to track usage for a single PC, compared with having several (uncontrolled!) members of the family each doing their own thing, and possibly accidentally focing usage over the quota allowed...
Next question is whether BT broadband is the best option? (I'm not pushing you to switch, but I feel so many people see BT as being a business they can trust - just like 25 years ago they said "No one gets fired for buying from IBM" - but it's big and pushy it's often not really the best deal. I just had a landline installed on a special deal - stay with bt for 18 months and the installation was free... but boy did they try to push BT Broadband on me too - had to say I had no need for the internet in the end just to shut them up!)
If you're locked into a 12 month contract with BT Broadband, and have a long time to go, then yes, look around for some software to monitor your traffic (or, depending on your router, there may be some free/ cheap s/w which will monitor it from the router statistics).
If you're "in sight" of the end of your BT Broadband contract, and could consider others, you may get a good deal offering higher limits, or not counting after midnight (if you use torrents). I think that for under 20 quid, PlusNet offers up to 80 GB a month, but they have a range of options, some cheaper.
I'm currently using the Three mobile network (with a 5 GB/month limit) but have been using different services from 500 Kbps in 2000 up to 2 Mbps in 2002/3 and then faster speeds after that. Clients of mine have used Bulldog, Eclipse, Tiscali, PlusNet, Namesco, Metronet, FreeUK, and, of course, BT. Personally I've used Freeserve (now Orange), Plus.Net, Eclipse and another, and even had 2 different ISPs on 2 phone lines for a while (!)
If you're locked into a 12 month contract with BT Broadband, and have a long time to go, then yes, look around for some software to monitor your traffic (or, depending on your router, there may be some free/ cheap s/w which will monitor it from the router statistics).
If you're "in sight" of the end of your BT Broadband contract, and could consider others, you may get a good deal offering higher limits, or not counting after midnight (if you use torrents). I think that for under 20 quid, PlusNet offers up to 80 GB a month, but they have a range of options, some cheaper.
I'm not using PN this minute (mobile contract expires in April, so have a bit of time to arrange and don't plan to pay two ISPs in parallel for the moment) but 3 friends and even my sister are using PlusNet, as well as some of my clients.
If you do think of switching, then www.SamKnows.com is worth using, to see which ISPs can offer you service separate from BT (usually lower cost and greater speed or quota allowed) and www.ThinkBroadband.com to check the good/bad points about different ISPs and see comparison of up to 6 based on user scores for speed, reliability and customer service.