Ideal Icos HE30 boiler start-up sequence

[Ulsinus]

My Ideal Icos HE30 boiler has been reliable starter since its installation 12 year's ago.
It has been serviced every year since by the original installer.
Today the boiler failed to fire though the thermostat is calling for heat.
I replaced the batteries in the thermostat.
I turned the electricity supply 'off 'to the boiler left for a slow count of 20 and then turned it back 'on'. The display then counts down 2-1-2-1-0. Then a barely audible click.
Then nothing.
I then press reset button for a slow count of ten but to no effect.
No error codes are displayed at any point.
For the avoidance of doubt I will now contact a Gas Safe plumber.
However, before I do so I would first like to understand the start-up sequence and what the root cause of the problem might be and how the problem might be resolved.
Therefore I would be grateful to receive the guidance and expert advice of the forum members.
Thanking you in anticipation.

 

[Aquarius]

Check your motorised valve.

 

[king of pipes]

Sequence is as below bud I have had the PCB ignition board fail on these and the main board go but I would advise a visit to do a few simple checks before shelling out more cash

 

[Ulsinus]

Sequence is as below bud I have had the PCB ignition board fail on these and the main board go but I would advise a visit to do a few simple checks before shelling out more cash

Thanks for the info regarding the start-up sequence.
Very helpful. My user documentation doesn't contain these. Least ways not so clearly.
I have now gone back as you suggested, shut everything down as before, and restarted from scratch.
The good news is that the boiler fired up. Heating ok. DHS ok.
Thank you King of Pipes and the others who have read and contributed to the tread.