fixa 24 Add 50' to MDA on a non precision approach.
Not true. That 50ft PEC is only applicable to DA's and not MDA's which means precision approaches. You can either use the aircraft's flight manual to figure out pressure error or use the 50ft addition as an alternative.
Jepp Terminal 2.6.2 Compensation for aircraft pressure error is not required when determining AOM (aerodrome operating minima) for non-precision approaches.
I would have thought the Area QNH given by ATS would have been more accurate
In addition to chimbu's response above, the area QNH given by ATC is also a forecast. There have been many a time when an area QNH delivered to me has been quite different to a/d QNH and when I questioned it, they have gone to the Sydney ADI or closest controlled aerodrome's ADI and passed that QNH on to me (mind you, I was transiting into that airspace)
Therefore the need to add the 50ft to the forecast terminal QNH MDA/DA (Jepps) if using area qnh