Massive standing charge inflation- why?

The Standing Charge is set by Ofgem in the Price Cap, to cover costs other than that of fuel. You can see what’s included here: Energy price cap | Ofgem. Changes over the past few years are shown in the chart below. The orange bit at the top of each bar is the cost of fuel, and everything below that is in the Standing Charge. This image is reproduced from this interactive Ofgem page.
  

There’s much more detailed information available at the Ofgem site.