rental charges backdated - is it legal?

it could be true but it has to be justified by invoices showing specifically the total consumption of the building and the quote of your consumptions and the advances that you made. Whatever he is asking for the past should be supported by documents, he can also increase the monthly provision for 2012/2013 but at the end of the fiscal year he has to provide a similar detailed breakdown of your advances and exact consumption.
If in doubt look online for Code du Logement, there is a section that covers the charges.