Hi mate only just seen this if it revs ok with the meter unplugged then 9 times out of 10 its the airflow meter motor factor ones are usually ok but you only get what you pay for if its a really cheap nasty one might be worth trying a better quality part..

Bmw's are a nightmare trying to get a fault code for air flow meter on the petrol cars its usually a air leak with hoses on the inlet split / breather pipes but on the diesels its not so common.

I would try a genuine air flow meter its a expensive test tho we have been bitten so many times with air flow faults.

At that mileage they are bad for not boosting getting to turbo mileage but that wouldnt stop it reving in neuteral would only notice that on the road under load.