Our neighbour had the same problem with Southern Electricity. He had been on Economy 7 since he moved into his house 4 years ago, although he doesn't have storage heaters or any benefit from being on an Economy 7 tariff. We telephoned the supplier and they are coming to fit a new meter and remove him from this tariff next week.
Your assumption is correct, if you don't have storage heaters or appliances that predominently run at night and not during the day, it would be cheaper to be on a single tariff. Economy 7 rates during the day are much more expensive. Your consumption history demonstrates that you are not reaping any benefit from Economy 7.
Put pressure on the supplier.