You probably won't have a hard time trying to rent long-term if you find the right landlord. I think that landlords may be amenable to accepting a lower price than advertised, even, if you were to agree to occupy the property for a year, maybe longer.
However, it seems to be the norm that flats are listed with more than one agent. I'm not sure why this is, but I think that most agents don't charge fees to landlords until they successfully secure a tenant. So, if you're a landlord, it is beneficial to have multiple estate agents at work on finding tenants.
Which is great for landlords but really stinks for flat-hunters if you put in an offer on a place and submit references only to find out a day later that the flat has been let by another agent!
(We *almost* ended up getting a fantastic one bedroom recently but were trumped by somebody else at another agency. It was surprisingly in our price range with a terrace, skylights, lots of closet space and modern fixtures and it was on a garden square near Hyde Park....I cried when I found out someone else got it!)
Flat hunting really $ucks, man! Try not to get bummed out...easier said than done, I know.... Bonne chance