The air flow sensor is at the back of the air filter box.
If you open the bonnet and look into the engine compartment from the front, the battery is all the way at the right. The black plastic box to the left of the battery is the air filter. At the rear end of the air filter is the air flow sensor. There is a plug at the right side of the sensor, just in front of the brake+clutch fluid reservoir.
A clip at the bottom of the plug prevents easy pulling out. Go with a finger of your left hand under the plug, when you feel the clip, try pulling/pushing it down while your right hand pulls the plug to the right. If you can't undo the clip with a finger, try poking with a small screwdriver. If that still doesn't work, you have to take the air filter box out. Undo the band or clip that holds the big rubbery hose to the back of the air flow sensor. That's the hose that goes down behind the engine towards the turbo and inlet manifold. Another big hose, where the air comes in at the bottom front of the air filter, is as far as I'm aware not fixed to the box so you should just be able to lift the air filter box off it. Turn it over so you can see the bottom of the air flow sensor plug, now it should be easy to unclip it and pull the plug. Put the air filter box back and reconnect the big hose at the back of the air flow sensor.
Go for a test drive with the air flow sensor unplugged. If the van runs better, the air flow sensor was causing a problem. You could keep driving with a disconnected air flow sensor, or get a new one.
If the van doesn't run better, the problem is something else.
What you describe about the van kicking into action at high revs, makes me think of a Turbo problem. I had that with the newer model of the same van, with 1.6 engine. The engine managment system habitually reacted to any (perceived) problem by opening the turbo waste gate, basically turning the engine into a non-turbo diesel. I've heard that this is quite a common annoyance with the 1.6. Haven't had the problem with the older 2.0 engine, but it might be worth checking it out.