[Stephen]

Yes every month you pay your bill for the coming month so if your billing date is the 11 of each month you are paying till the 11th of the next month. however as you upgraded on the 28 it is charging you at the new speed for the rest of the month and also your usual month in advance. as well as the part of the month you had at the lower speed.

if you leave it at the 1.5MB then next month bill will go back to £37.99 or if you called to downgrade than you will get a credit for the more expensive tariff.

Hope that makes sense,