If possible, tell the technician in advance about how the boiler has been performing (rather than asking for a service when the boiler has broken down).
The first thing a technician will do is turn the boiler on and check the controls. Informing the technician of any possible noises, anything unusual or electrical work just completed will indicate possible areas where a problem might have occurred.
Pressure jet oil boilers do not require the extensive range of parts needed by gas appliances, so there is a good chance that we may carry spares. Before you call a technician out, do the basic and check that nothing is turned off that should be turned on. For oil installations check that you haven't run out of fuel, sounds silly but it happens. Check that the programmer is working correctly and that there is a demand for heat at the boiler. Read the boiler manual left with the appliance.