I put in an EGR blanking plate to see if that was contributing to the problem, but it made no difference to the MAF readings (as I suppose you would expect). I'm going to see if I can visually inspect the hoses for leaks, but I'm not sure how much access I'll be able to get at home.
It's odd that we are getting such similar MAF readings. Maybe your black smoke isn't related to the MAF?
I'd also like to get a definitive source on the expected MAF readings because I don't trust the AI sources too much. They tend to synthesise information from multiple sources.