Thank you,
@Johntheo5 that's what I wanted, basically to know what was normal. In my mother's old house with a modulating boiler, set the TRV to 20°C and the room was (reported at the TRV head) 20°C +/- 0.5°C and once room was at temperature the radiator just stayed warm, there was to all practical purposes no hysteresis.
I note your
but because I've no pump overrun, the boier temp will rise to ~ 90C after boiler cut out
and was told by the heating engineer who helped getting the system running, that this was the advantage of the C Plan in that on switch off heat went into the DHW.
One needs to look only at the general picture of comfort which takes in more factors, some indefinable.
Yes, I see what you mean, had on moving in the central heating worked reasonably then it would have never been touched, I would have just accepted my lot so to speak. I was water flowing in reverse, needing to go outside to turn it on/off, and no auto method to turn it on/off, which resulted in the adding of motorised valves (to stop reverse flow and heating pipework of sections of house not in use, i.e. main house and flat) that I added a thermostat, think there should have been one, found the receiver, but that it seems was not wired up, one had to plug in and unplug pumps to direct the heat to part of house where wanted.
I found wiring and a programmer,
but found the triple and earth changed colours end to end from red, yellow, blue to brown, black, grey and one core open circuit, not a clue where joined, so it was how does one control heating and DHW with two wires, and power the boiler, my answer was to use a local supply to boiler, and used Nest Gen 3 (happier with only 12 volts on those wires) so the main house hard wired and controlled from the thermostat in the hall, this did not work very well, hall cooled too slowly. So heating worked now, but needed improvement.
I tried the balancing, fitting electronic heads, but it was not until a second hub was fitted, this time Drayton Wiser, in parallel with original, that I started to get reasonable heating. But still nothing like the gas fired heating in my late mothers house, but the question is, how good can a boiler that turns off/on get? Mothers had a modulating boiler, this house simple on/off.
I have realised there is a problem with radiators under the window, as the TRV is on the radiator, so is being cooled by a cold outside wall, the Kasa and Wiser TRV heads can be linked to remote wall thermostat on the inner wall of the room, but not the eQ-3 and Energenie, the flat still has mechanical TRV's so could replace some in main house and reuse them down steps in the flat. And/or fit remote sensors to the two existing TRV's which will work with them. But as
@oilhead points out, never be perfect, and at some point need to say that's near enough.
@Harry Bloomfield has pointed out, the Wiser is working out when heating is wanted and how long it takes the room to warm up, and switching the boiler on a bit early to allow for warm up time, the Nest Gen 3 thermostat rarely gets cold enough to turn on heating, but left in case I want DHW from the oil, and as a back-up should the Wiser thermostat battery become discharged.
I note I had to set the controls to oil or gas, and would assume a longer mark/space for oil compared with gas, so is this as good as it gets, or is there still room for reasonable improvement?
It took some time to work out what the graphs are but 
found it in the end, the green bit (last graph) is outside temperature, green line actually temperature, and black line the set temperature. The first graph is from the Kasa TRV head, and does show the hysteresis, which looks massive, 18.2°C to 25°C, but this is under the window, where we sit it not as much variation. 
