lost screw for automatic air vent on baxi 105he

I have a Baxi 105he and yesterday it stopped working and we found a leak coming from the pipe at the bottom of our downstairs radiator. We tightened the valve and the leak stopped. Bled the radiators from 1st floor too second floor. Whilst other half and brother in law were up in the loft trying to re pressurise the system fo some reason they loosened the screw on top of the automatic air vent and lost it (!!) Does anyone know what size it is, where I can get a new one and how important it is? Can I have the heating on or is it best to wait until the screw is replaced?
Would appreciate any help at all thanks ever so much

The screw on the auto air vent is meant to be left loose and if there is no water coming from it then the valve is working ok and you can use your boiler.Once the air is vented from the auto air vent the float inside rises and closes the valve.The screw is only use during repair work to boiler or system.

Thanks so much for your quick reply....Will put it back on now.