can any one help with ecu problem i have???

car 2000 vauxhall vectra 2.0 cdx estate

the problem

engine management light comes on when the car is ticking over but
goes out when moving,when pulling away from standstill the car has a
quite significant flat spot also if i turn on the heater when the
car is idling the revs drop and the car almost stalls, i've taken
the car to my local garage and his computer says the maf has a low
reading,i have replaced the maf with a brand new siemens unit but the
fault remains and the computer is giving the same low reading the
garage owner says it is probably a ecu problem but he can't be sure

there is no problem starting the car from cold or warm and the only
fault showing up is the problem with the maf

has anybody any idea where the problem lies

also can anybody tell me where the ecu is located in the car

regards

melvyn brown