question re billing

[#1]

If I go just a teeny bit over my 10gb, then you top me up with another 5gb and add £5 onto the bill...what if I only use say 1 or 2 gb  - does that carry over to next month ? or is it a standard charge of £5 whether I use 1 extra gb or all 5?

[#2]

You lose all the surplus usage even if you only use 0.1GB

[#3]

Quote from: ming

...what if I only use say 1 or 2 gb

Which is all I do every month. Should I qualify for a discount? 

[#4]

Guys...
If you're on Value then you can upgrade to Extra for just £5 per month and get another 50GB
Jojo