Myson Midas central heating boiler problem

I have an odd fault on a Myson Midas Si central heating boiler. It's a
very old boiler which I manage to keep going by changing parts as
necessary. The latest fault is this: The boiler has an insulated tank
where it stores a small amount of hot water so that when you switch on a
hot tap, hot water is instantly available (it doesn't actually work that
well but that's the theory). Every few hours the boiler will fire to
heat the reservoir stored in this insulated tank. I usually turn the
temperature dial that controls the hot water down to it's lowest setting
when I go to bed as it can wake me up in the night. When the boiler
tops up the reservoir it always used to run on a low fan setting which
was much quieter but lately it has started running on high fan only (as
if the actual hot tap has been turned on). I suspected the hot water
inlet sensor may be faulty so have changed it but this has made no
difference. I have also changed the hot water flow sensor but again
this has made no difference. I tried swapping the main control board
and air flow sensor have spares for both but this has not cured the
fault either and am now running out of ideas. The only other board I
can change is the ignition pcb which does also seem to link to the fan.
I don't really suspect this part but am wondering if anyone else had any
ideas ? Interestingly, the fan runs on the low (correct) speed if I
leave the water temp setting on high - this means the boiler will run
more frequently to keep the reservoir at a higher temperature of course.

Any ideas appreciated..

mine kicks in every hour or so to heat the tank is this acceptable

is this brand new has it just started doing this.

i know its always done it but have been having sleepless nights latley due to stress so have started looking at the times between kick ins dont know if it should be longer intervals,