Short answer
Commercial boiler ignition faults fall into three groups: no spark (electrode, lead, transformer, control board), a spark but no flame (gas supply, pressure, air in the line, gas valve, air/flue blockage) and a flame that is not detected or drops out (ionisation electrode, earthing, combustion setting, flue recirculation).
Splitting the problem
Manufacturers' manuals diagnose ignition lockouts this way — for example Remeha's E14 and Ideal Evomax 2 faults 52–58. Working through spark, gas and flame detection in order finds the cause faster than replacing parts.
London-specific causes
On older London installations we often find undersized pipework or meters that cannot hold pressure when a cascade fires, and condensate pipes that freeze in exposed roof plant rooms in winter.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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