Private landlord's - Where are you?

I've been looking to rent a 3 or 4 bedroomed property in Portishead for the last month - this should be an easy task (you would think), especially when I offer to provide several months rent in advance, but it would appear not.  The minute housing benefit is mentioned letting agents have no interest.

I can find no private landlords to rent from (minimum of 3 bedrooms required). 

Does anybody have any ideas?  I've placed adverts on various forums and classified sites but no success?

I'm now totally desperate!