Echo oilman's thoughts, they are simple, do the simple things right!
The only probem generally with float controls is the screwdriver in the calibration screws.
Moving the unit outside is a mistake as in cold weather the oil can thicken sufficiently to cause problems in the feed pipe from the unit to the burner especially when it has to overcome the extra resistance to flow causedby lengthening the run with only a 6mm head. 'Going out' is a classic symptom of too low a fuel delivery. Have you checked the pipe from the tank to the float control? Isolate the tank, disconnect the pipe from the float control and turn the fuel back on. Let a good 2 - 3 litres of fuel through to ensure there is no blockage. If the flow slows noticably, that is where your problem lies. Artificially filling the burner to start it should not be necessary. There should be sufficient level in the burner after 10-15 mins to light the burner.
thanks, this seems the right thing for me to do. will need to move the unit back in again anyway.