It's a Potterton - around 5-6 years old.

Although the radiators are hot, the boiler still emits a similar
pattern when heating just the central heating, i.e. on for a short
period, off for a short period. I may be wrong, but I think the boiler
used to fire for much longer when heating the radiators before the
flame going out.

Would that still point it to the diverter valve?