problem with central heating

turn on c heating in hot press - hum starts but boiler does not fire?????? Sometimes does sometimes doesn't - any suggestions?

Are you turning on a gas boiler, or the immersion heater in the water cylinder?

oil fired central heating - switch in hot press

Is there a thermostat that is stopping this? With the weather being so hot perhaps that is the culprit?

When had the heating system been maintained the last time, by a real plumber? This should be done twice per year.

serviced in march by a real plumber, will have to call him again it seems

Get the meassuring protocol from him. It should state the NOx as well as the SO2 and the CO (and if possible the CO2) values. These you have to compare with the manufacturers values about your boiler. If you have an oilburner(?) then it is very important to get the sooth output meassured as well. This is done usually with a piece of filter paper that turns black (more or less) when exposed to the fumes. The blackishness is compared with a standardised chart and a mark is given by the protocollant/plumber.Keep the filter and the protocoll for future references, to compare it with the next meassurement.