Looking for holiday apartment in Venice

Hello

We are two very nice British couples, one currently living in Paris, one in Barcelona. We're coming to Venice for the Biennale and are looking for an apartment for 3 nights (1-4 October). We'd prefer a 2 bedroom flat, nearish to the Biennale site and outside space would be nice - all for up to 700 euros! Does anyone have a place to rent out or could you recommend any agencies or private flats you've had good experiences with?

Many thanks