Best answer by woz
It sounds like your estimated annual usage is too low so your payments have been set too low (I'm assuming that the statements do have some real readings on them and not all estimated).
No problem to get this fixed, go to Help in the app, ask wattbot, type "message the team" and ask for your direct debit to be amended to bring you back into line. Replies aren't instant, a day or 2 + easter.
Your account should be approx one month in credit if you pay the dd when you get the statement, so PP will adjust to bring you up to the correct usage and add the shortfall when they calculate your new DD.
If your account has been running more than 6 months this should have been picked up, but you don't say over what period this has happened, or how much your payments are which would make a difference.
The lower summer payment is irrelevant because if the usage was on track with your initial quote it would have evened itself out over a year.
Your payment will likely increase substantially but then I don't know how much it was to start with.
Our bills have accumulated a debit of over over £500 debit at present. How are we going to pay this if you are going to charge us far less in the summer months than we are presently paying and how on earth do we contact pure planet directly when we have a serious concern about our monthly bills which we believe are seriously being undercharged.