Airlock in Potterton neataheat 10-16?

I recently drained the whole system and the boiler only comes on for about a minute, overheat and cuts out. I have down drained the system 4 times and cannot shift the airlock. The pumps new, the rad's are fine. Can I do a pressure flush to remove the airlock? I cannot find any form of valve on the system except upon removal of the front lower cover whereby there is a hose take-off point that just drips out 'muddy water'. Next to it (left) is an angled pipe with a standard screw head but also a hexagonal nut. If I undo this, does it bleed the boiler? As yet, I have not removed the nut because I am nor familier with what it's for. Any help would be thankfully received. The boiler was working quite well before I drained it although the rads, especially up stairs never got really hot, hence the drain. Thankyou.