My monthly payment is £40.00. However, each month I spend less than my monthly payment. Therefore I am in credit. Why bulb suggests to top up my monthly payment to £51.00?

It is likely your usage will rise as we come in to the winter months, The monthly payment is arrived at by taking a years usage and dividing by 12 to get a monthly payment. If your credit continues to rise they can credit some back to your bank account but they do expect you to retain a months dd to cover the following months payment.