The thermostat is on.
The pump is running and it's nice and clean inside.
The motorised valve seems to be working fine.
The hot water tank is fine (except that it's full of cold water at the moment).
The tanks and valves in the attic are also working fine.
No trapped air in the radiators.
No trapped air in the boiler.
No funny noises, banging or "kettling".
The boiler's flue is clear and its fan is spinning happily.
The boiler's ignitor is sparking correctly.
The boiler's pilot light is lit.
But the burner will not fire up. The pilot light stays lit and the ignitor keeps sparking and clicking away. Once or twice the burner did fill up with flames, but within 20 seconds it had died off again.
Here's a short video clip to show you its behaviour…
What do you think is wrong with it?
And how can it be fixed?
All thoughts and suggestions are welcome, but please don't waste your time typing "Get a professional" and "Time for a new boiler" or "Safety first!" – that kind of nonsense doesn't help anybody.