As this is your first post, welcome to the Pure Planet community! You are talking to your fellow customers, and they are a helpful bunch, as you have already discovered through angelabikerbabe's answer.
She has told you how to contact Pure Planet to get your monthly payments changed. Remember, they check your account every 6 months, and will suggest a change if they think it is needed. But if your negative balance has increased each month for two or three months in the summer, then you may need more urgent action. Have a look at your last two or three statements to see if that's what's happening. Then you can also check that your last statement has included your direct debit for this month. I'm just wondering if you have been caught by the direct debit being held up by the weekend, which can mean that the statement is sent to you and doesn't show the latest payment, which makes it look like you have a sudden debt.
If you have got this far, here's my final point - much much better to stay with Pure Planet and sort it out, than switch. If you decide to switch now, it will be harder to sort out, you'll still have to pay what you owe... and the chances are you will end up paying more.
Hope that helps. Contact Pure Planet if you are still worried, or feel free to ask more questions here, people will be pleased to help if they can