Thank you for your post. My apologies for any confusion with this.
The way data charges work for our Mobile customers are as follows.
You will be charged £3 per 1GB used while you have no data allowance left.
For example if you exceeded your data allowance and then used any amount of data you would be charged £3. If you then used up the 1GB of data and then used more you would be charged another £3 for access to that 1GB. Do bare in mind that the 1GB allowance will only last until midnight on the day it was added.
If after you used the 1GB of data and didn't use anymore until your allowance refreshed you would only be charged the one £3 charge.