Yes, there are five wiring diagrams for the back-plate.
It is unlikely you are using either of the OpenTherm options, with Hive terminals marked 12 OT at bottom left, and if hot water then will not be one channel, and don't think Hive make a three channel version, that seems only to be used with the higher quality Wiser hub, so with two channel a wire from L to 3 will turn on the DHW.
However, I have looked at how long my boiler has to run for DHW, mine does not modulate so 20 kW at 20 minutes a burn, 6 times a week, approx so 40 kWh per week used, and the immersion heater in my case uses excess solar, which means it has a meter showing how much used, and I use around 12 kWh per week. Since my off-peak is 8.5p/kWh I am far better off using electric, and even without having off-peak, the cost gas/oil to electric is so close, due to reduced losses with electric, they likely work out about the same.
Using a boiler in summer (winter is a different story) means you heat the boiler and pipework up each time it fires up, which will have cooled down before next firing, with my old instant gas boiler, I could fill a bowl with water before it got hot on Eco setting, i.e. no stored hot water, and half a bowl with Eco off, so showing it takes the energy to heat half a bowl of water, just to heat up the boiler, without looking at the pipes boiler to hot water tank, as to gas being cheaper well it may be, cheap enough so using gas or electric works out about the same cost, but using gas also warms the house more, not wanted in summer, specially where hot water pipes not lagged, so now I use the immersion in the summer.
This time of year I would generally check how much oil left, to see if I needed a top-up before the winter, it would be around half full, (600 gallon tank) this year 3/4 full, so not using oil for DHW has saved me around 150 gallons of oil, clearly can only be approx, but I used gas, then oil to heat DHW for years as thought it was cheaper, then found I had been wrong, and in my case using electric is far cheaper to using oil or gas.