Sudden increase in usage after electrical work

[UNG]

I agree it's Combi boiler, so water cylinder no longer required.
But did the installers of the new boiler remove the cylinder ?
Has the old immersion heater been powered by the CU replacers ?

Not sure why your pushing the immersion heater route even if one had been left connected I would think it would have burnt out by now as the water evaporates from the cylinder also there would likely be a noticeable increased level of humidity / condensation in the property