The average household fuel bill is now around £1500 per year, so that's a bit over £120 per month. So your electricity usage since your new meter was put in is
remarkably low. Indeed the £80 per month you were paying before was still far lower than most people pay. So I doubt that you've been overcharged anywhere. (If anything, it seems more likely that you're currently being considerably undercharged!).
The regulator Ofgem wouldn't get involved until you'd initially complained to nPower and then had your complaint rejected. It's hard to see how you could make out a case against them at this point, as no independent testing of your old meter could be carried out. However, if you want to pursue it further, here's the OfGem information:
https://www.ofgem.gov.uk/consumers/household-gas-and-electricity-guide/complain-about-your-gas-or-electricity-bill-or-supplier