Additional usage

[#1]

I just want to check how the additional usage thing works.
I've done a lot online this month so am approaching my limit quite early on. I understand that anything over the limit is charged at £5/5GB, and I can set my own limit on that. What I want to check is: do I actually have to set a limit in order to "activate" or authorise the extra payment? There is currently no limit set, so will I just be automatically charged for each extra block of 5GB I use, or will my speed be cut until next month unless I set a limit?

[#2]

If there is no limit set then they will keep adding 5GB blocks for ever

[#3]

thanks 

[#4]

but if you go over at 2 minutes to midnight on the last of your days, it will add 5G and it doesnt carry over