Having started Feb with 2 units carried over, you can use 6 (your tariff) before excess charges but any over that will be £5.64/unit. You'd then carry forward the original 2 over to March
You can use a further 10 before excess charges as you are allowed to go over by 6.
It's only when you go over -6 GB in the billing period that you would start to be charged.
If you used 9 in the example above you would start the next month (March) with 1 (-5+6) and be allowed to use a total of 7GB before being charged etc....