When we were using gas we used about 0.37kW/°C for heating.

During the period 12 Feb 2021 to 11 Feb 2022 we used 16092kWh total electricity (no gas).
Base usage (non-heating) was 13.55kWh/day = 4946kWh/year.
So energy for heating was 16092-4946 = 11146kWh/year.
Cost for heating at 14p/kWh would have been about 11146*14/100 = £1560.
Cost for base usage would have been about 4946*14/100 = £692.
Standing charge for the year was 365*23.81 = £87.
Total cost for year would have been about  1560+692+87 = £2339.

If we had used gas, we would have used about 11146*0.37/0.20 = 20620kWh/year
for heating.
Cost of the gas (at 4.57p/kWh) would have been 20620*4.57/100 = £942.
So we paid about 1560-942 = £618 extra for heating by using electricity. This is an extra 618/942*100 = 66%, when comparing heating costs alone.

The total cost using gas has to include:
Annual service of gas boiler £80/year
Replacement of boiler every 10 years £200/year
Gas standing charge: 365*26.59/100 £97/year


Total additional cost £377/year
                       

So the total cost using gas would have been 942+692+87+377 = £2098.

Taking these savings into account we are paying 2339-2098= £241/year extra by using electricity for heating.

This is 241/2339*100 = 10% extra (relative to electricity total cost), or 241/2098*100 = 11% (relative to gas total cost).

We are actually still paying the gas standing charge.

Using modern boiler

The difference in the heating slopes (0.20kWh/°C for electricity and 0.37kWh/°C for gas) is probably partly due to the inefficiency of our old boiler, and partly due to more effective zoning using electricity. If we assume the worst case (i.e. the difference is entirely due to boiler inefficiency), then the boiler efficiency would be 0.20/0.37*100 = 55%.

A modern boiler should be capable of 95% efficiency, but in practice they are not set up correctly and an efficiency of around 85% is probably more realistic. This would give an energy requirement of 0.20/0.85 = 0.24kWh/°C for heating, i.e.11146*0.24/0.20 = 13375kWh/year.

Annual cost for heating would be 13375*4.57/100 = £611/year, and total cost would be 611+692+87+377 = £1767.

So compared to a modern boiler we would be paying 2339-1767 = £572 extra using electricity.
This is 572/2339*100 = 24% of the total cost (relative to electricity total cost), or 572/1767*100 = 32% (relative to gas total cost).

Using current tariffs

If we were to switch tariffs now, the rates now (15 Feb 2022, from Octopus) would be

Using electricity, the average rate would be about (30.61*20+5*4)/24 = 26 p/kWh (increasd by a factor of 26/14 = 1.86.
So cost of heating would be 11146*26/100 = £2898.
Base cost would be 4946*26/100 = £1286.
Standing charge for the year would be 365*24.11/100 = £88.
Total cost for year would be about  2898+1286+88 = £4272.

Old boiler

Using gas with our old boiler, the cost of heating would be about 20620*10.33/100 = £2130, and total cost would be 2130+1286+88+377 = £3881.
So we would be paying 4272-3881 = £391 extra for using electricity.
This is 391/4272*100 = 9% extra (relative to total electricity cost), or 391/3881*100 = 10% greater (relative to total gas cost).

Modern boiler

Using gas with an 85% efficient boiler, the cost of heating would be about 13375*10.33/100 = £1382, and the total cost would be 1382+1286+88+377 = £3133.
So we would be paying 4272-3133 = £1139 extra for using electricity.
This is 1139/4272*100 = 27% extra (relative to total electricity cost), or 1139/3133*100 = 36% greater (relative to total gas cost).