Han did the tappets become noisy when the buzzer went off, if so you have genuine low oil pressure if not it's probably a sensor.
If you do have low oil pressure ask the garage to whip the sump off and check the gauze on the oil pump pickup for blockages. The missus' mk2 developed a low oil pressure problem and the buzzer kept going off I cleaned all the carbon crap off the gauze and it was fine afterwards.
Defiantely worth trying if funds are running low.