Worried about something

[#1]

hiya my internet it up wooooo, just a few questions i am on the 5.99 pound a month contract i am a bit worried i am gonna go over, if i put the plan up to the 11.99 one will i be able to carry over my 4 months free?.
also i am a heavy downloader of movies on my ps3 but i can do this at night time, how reliable is the unlimited night time downloading if i download at night might they take it off my 10 gbs? i mean not on purpose but the system cant tell when its free ect and take it off my 10 gbs. basically how reliable is it if im gonna survive for a month with 10gs i dont want to eat it all up at night when it is unlimited.
Also 10 gbs is just downloading it doesnt include uploading right?
thanks alot

time to try world of warcraft for 1 hrs and see how much mbs i use in one hour

[#2]

Easy parts which I can answer
The overnight download monitoring makes decisions in your favour - some of the non free  time may appear as free but not the other way round
The 10GB includes downloading and uploading

[#3]

if you click the view my usage button at the bottom left of this page, it will give you a chart of this month usage, note you put your main username and password in not the forum one.
I shows peak and off peak figures