Upgrade...

I wish to upgrade to the 30 day plan offering 30gb of data for £10 a month. When asked for payment, why is it asking for £10 when I'm already on a plan that costs £7.90 per month? In other words why doesn't it charge me £2.10 & then take £10 per month off me thereafter?

I would presume you’d need to cancel the £7.90 per month plan,and start again with the £10.00 plan. That way the details of your tariff would change on your account. I’m guessing there’s no algorithm in the system to just take the £2.10 difference. All the best