you've overfilled the boiler so the pressure relief valve has opened up to dump the excess water out of the back of the boiler but the valve has then failed to shut properly because dirt in the system has got stuck under the valve seating.
not a major problem, we can normally open and close the valve a few times to flush some water thru and hopefully wash the dirt away, this normally works but on the rare occassion it doesn't we just replace the valve for a new one.
as for your pressure dropping in the first place it sounds like you need your expansion vesesel repressurising, again not a big job.
i cant understand what you mean when you say the engineer told you to turn 2 screws to release the pressure? if you mean you opened the filling loop valves then adding extra water is going to increase the pressure, to reduce the pressure you need to drain some water off from a radiator.