Thats interesting because (assuming a room(s) temp at 20C) that your rads are effectively "T36" which emit 65.2% of a T50 so, your rads output, 35.5*65.2%, 22.1kw which is probably why the LLH gave similar outputs with a boiler target temp of 75C but secondary temp of ~ 65C and your d0 setting of 20kw was sufficient to allow the boiler to reach its target temp of 75C.
If you didn't have that LLH to hand one wonders was one required in the first place?.
If you didn't have that LLH to hand one wonders was one required in the first place?.