I can't get a reliable wifi signal between my Airport Express in the office and Express in the house and i can't get a cable between the two. Any suggestions for getting internet connection in both buildings?

I need to get internet to both the house and my office, which is in a separate building approx 20m from the house. I was advised by Apple to buy an Airport Express, set it up on one or other building, and connect wirelessly to the other. Tried that, but couldn't get a reliable signal.

I was then advised to buy another Airport Express and use it as an expander in the other building. Trouble is, i can't get a reliable wireless signal between the two Expresses, so it keeps dropping the connection, and is slow when it is connected.

I rang Apple again and was told that i could try a third Airport Express between the two existing ones, but that is impossible as there is only outdoors between the two.

It would be very difficult and expensive to get an ethernet cable between the two buildings (have to cut through concrete etc), and I see from the discussion forums that an Airport Extreme has the same wireless signal as an Express, so replacing one of the Expresses with an Extreme seems unlikely to resolve the problem. Can anyone think of any other options?