Best answer by Duppy
A couple of things you need to clarify
Is your £6.99 balance before or after your monthly direct debit payment. You should be approx 1 months payment in credit as we pay in advance of our usage.
Is the £50 increase due to the winter payment increase, if so that would be normal for that amount.
Any other increase in direct debit should be agreed with you and would be calculated to pay for your usage plus any negative balance, to bring you back on track.