Somewhat like you, I have an allotment without a water supply and have to carry all my water from my own garden tap about 200yds away in watering cans..so I keep watering to a minimum! However, I recently saw a neat device on the BBC programme Dragon's Den. An inventor (Guy Unwin of Planit Products) has come up with a collapsable 80L plastic container called H2GO which fits into a wheelbarrow and so can transport a good quantity of water in one go. I have just bought one for £9.99 (postage free!) from e-Bay. Search on "H2GO 80 litre" and this will pick up the e-bay shop.

The Mole