Sounds like they might not be the right ISP for you as it seems you use a lot of peak bandwidth. I'll try and briefly explain how their charging works.
They give you 2 units of bandwidth per month (you can set this to more if required). This is £6 a month (ex vat).
This usage is then used differently depending on the time of day you use it (they only meter downloaded traffic, not uploaded).
Peak Time (Mon-Fri, 9am-6pm): Rate is 2Gb/unit
Off Peak/Weekend: 100Gb/unit
Night Time Special (2am - 6am): 1Tb/unit
So if you use 10Gb peak, that's 10/2 = 5 units.
10Gb during off peak would be 10/100 = 0.1 units.
10Gb during night time special would be 10/1000 = 0.01 units.
Ontop of the bandwidth units, there is a base charge for the broadband connection, currently £10.17 ex vat per month for a 21CN connection. And if you are doing the line rental through them, that's another £10 a month, ex vat.
So for me, total is: (10+10.17+6)*1.2 = £31.40 per month (inc vat).
If you are keeping the line rental with BT, it would only be £19.40 per month (inc vat).
They also role over unused bandwidth units each month, up to a maximum of the base amount you've asked for. For example, at the start of this month, I actually had 4 units to use, as I had a quiet month last month and didn't use my connection much.
They also role the other way too, so I believe you can go over your base usage for 1 month and not be charged extra. If you do go over your base usage for the 2nd month running, they then charge at a marginally more expensive rate for the number of units you've gone over (if you keep going over usage, it's advisable to up the base usage of 2 units to something higher that matches your 'normal' usage amount).
At 2Gb/unit during peak time, that is easily eaten up if watching video/downloading - I've noticed I can eat through that very easily if I'm at home during week days. Sounds like this would happen with you, so I would suggest looking at alternative ISPs which allow more peak time traffic per month, and who more closely fit your requirements.
Edited by mixt (Thu 16-Jun-11 16:57:27)