Roaming.

[Si142]

I have a number a planned business trips to Europe of about 8 days each. I understand that using the £10 add on would be most convenient. 

Questions, 

Does the add on stay on until the end of my contract? Am I able to add it for a single or a number of months only?

[bristolian]

You can add the Roam Abroad pass at any time, and then remove it at any time.

Charges & credits are applied pro-rate when you apply or remove it partway through a bill cycle.

[XRaySpeX]

The Roam Abroad Pass is charged pro-rata from the day you order it (text ROAMING PASS to 150) to the day you cancel it (text STOP ROAMING PASS to 150). You do start by paying up to the next bill date. Then in the billing month you cancel it you will get a rebate for the days after you cancel.