How should one deal with intermediaries when looking for housing?

No way...  Seriously...
You must to have a Moroccan friend who you can trust. And let all bargain and payments to him/her.
Me and my friends tried to deal many times, all failed. When we take a local friend with us payments decreases in half.
This is something tried maybe more than 7 - 8 times. Result never changed.

Thanks for the advice!

Actually, I feel sorry to not be able to give a usefull tip but this is what we all faced.
If you decide to handle yourself, I just would like to add a few detail;

If you deal over a professional real estate agency you will pay comission of one month hire (never more)
If you want to get advantage a bit, take the "first offer" of the real estate agency, after tell them that you will pay their comission over their first offer, but you need a discount in hire. That works. It makes them to not worry about their earnings and try to convice you with discounts. (tricky?  yes : )

If you deal over a comissioner without a legal company, the comission rate is 50% or 35% of the price. They will ask full hire but be sure that they are already ok with the amounts that I wrote. (at least like that in casa)
And the same offer of comission / discount also works.

One thing about Morocco, in bargains repating offers changes results. You start from 20%, he will say oh nooo and offer 60%. You say ok 25% even works, he will say no no lets make it 50%

After a few minutes you met at a point around 40% - 35% Ok sounds awkward but it works. Don't hesitate, this is somethng usual in dealings. And this discussion will complete with smiles. Moroccans are hot blooded and sympatic. They just try to earn more if possible, and sure this is quite normal.

Again, great advice, thank you. Intermediaries are a new concept to me. I thought they worked for the landlord and was giving them money for showing me apartments, I didn't realise they charge a fee when you accept an offer (it came as a shock!) I am still looking, so will be more careful who I speak to. Many thanks!