Hi TLDenise, though I think Mayfair and Marylebone are wonderful neighbourhoods to live, they are prime locations and two of the most expensive areas in the London. In my opinion, it will be very difficult to find a place for £900 anywhere within Zone 1 other than a studio, and that will be tough, and unfortunately it’s getting harder to find anything under £1000 in many parts of Zone 2 as well.
But as others have said, the embassy is centrally located so if you’re willing to commute you certainly can find something decent for that price further out on the Central and Jubilee lines on the tube, the stations closest to the embassy. Green Park is also nearby and that will give you access to areas on the Piccadilly and Victoria lines as well.
As Ophinky suggested, if you let people know what the types of things you’re looking for it will help narrow the search.