You could have air in the oil pipe. You need a 4mm allen key to undo the screw marked P on the black oil pump. Put a tray under it first. Turn the boiler off to avoid surprises.
Once the screw is out, oil will come out with bubbles, then when the bubbles stop put the screw back in. I suggest you wear gloves, oil stinks.
If you haven't got an allen key, but you have a 14mm (maybe 15mm) spanner, turn off the oil tap on top of the burner box. Undo the flexi pipe from the oil pump, stick the end in a pot and turn on the oil. When oil flows without bubbles, turn off the tap, replace the pipe, do it up tight and turn on the tap.
That might do it.