Steve, I just got back from a road trip through West Africa, so my experience with travel insurance may be relevant...
I finally settled on a UK company called STAYSURE - "over 50's experts" - who do travel insurance for expats. Their rates are very competitive, but as I had no cause to make a claim, I can't say how good they would be if anything had gone wrong. I bought their annual cover, which includes emergency repatriation.
Before leaving, I had several email exchanges with them about cover in Mauritania. (Morocco, Senegal and Gambia were not a problem.) What it boils down to, with STAYSURE and any other insurance company, is that if the FCO advises against travel to a region/country then whatever your policy says, you are NOT covered there. I posed the scenario where I fell over in a shower in Nouakchott and broke an arm; the answer was that you have been told not to go there! Doesn't matter if you are blown up by Al Quaida or if you fell in the shower - you shouldn't be there!
As we were only going to be in Mauritania for about 24 hours, I took the risk.
Hope that helps!