I had a monthly 60GB plan for £10/month. Just after it charged me for another month, I decided to upgrade my plan.
The new plan is an annual 100GB/month, for £10/month, with the first three months free.
I was sent information to say that I had used all roaming data and would be charged in future, though I have used no roaming data at all. I have also been charged for another month though I should not have been. I should be getting almost four months without payment, since I had just paid before my upgrade.