Contrary to what has been stated above, my understanding is that the boiler ignites at close to MAXIMUM power and only turns down to the set value of d0 after about a minute.
Via observation, that's the conclusion I had come to, I'm glad someone else believes that's the case. It's partly the cause of the problem we are having, I'm sure of it. Because the heat is not dispersed quickly enough into the cylinder, partly because of the short circuit and partly because of the in-efficient transfer in the cylinder, the boiler never manages to reach the modulating (less than full power) part of it's cycle.