Combi boiler won't switch to low flame/modulate

[MaxNix76]

Hi all, I hope you're all well. I have a problem that could easily be turned into a 5 series TV show and was wondering if anyone could help.
I"ll try to keep the saga as short as I can.
Combi boiler Potterton Promax 28he. Had an error message that needed looking into and because in my area not many independent boiler engineers are present, I used the home emergency services included with my Lloyds bank account.
23 visits later (half of them to diagnose, the other half to carry out the work), they replaced for me:

- boiler fan
- gas valve
- PCB
- all valves
- all thermostats
- ignition coils
- sensors
- pump
- diverter valve

I mean, chuck a new shell and I pretty much have a brand new boiler! So what's the problem?
The damn thing gets to temperature when the heating calls for it, and then cuts off and on, off and on CONSTANTLY! It doesn't modulate.
The weird thing one of the last engineers managed to prove is that if the boiler is drained completely, then re-filled and allowed to operate for the heating FIRST, then everything is perfect, low flame once at temperature, no problem. But once the hot wanter is called in for just ONE time, then closing the hot water tap returns the boiler to the off/on non-modulating issue.

The engineer's company is totally stumped; more than 15 different engineers have looked at it, they discussed it on their internal forum; no one has any idea of what the heck is going on.

Has anyone ever encountered anything like this?

Thanks in adcance

[Someone-Else]

Hello MaxNix76, Long story, short version, we have several gas engineers on this forum, but you really need someone to look at your boiler (Can't do that from a keyboard) and those are too busy to post this time of year, so I can only suggest you call Potterton direct, or failing that British Gas.