When you say they have been 'overcharging' you, do you mean they have overestimate dyour readings or used too high a tariff? If the former then you should contact them and ensure you submit regular readings in future, whichever supplier you are with.
The process is still flawed in that comparison sites overstate savings (in that they assume you'll move onto a higher variable tariff if you stay); and renewal notices are deliberately misleading. Mine from Together Energy showed my unit prices going up by around 15% but they claimed my bills wouldn't change- but on closer nspection they had used actual usage for the previous year and then used 'a typical customer's usage' for the projected new cost. I switched to someone with lower rates.
I have spreadsheets showing monthly readings, daily costs, dual fuel discounts etc, and I am pretty numerate, but i still find the comparison process difficult each year because of the different ways they show figures (eg some with VAT, some net of VAT)