Sprinkler system leaks, main valve leaks must fix pipes under h2o?

It sound very much like this system is well past its best before date. Before you can even consider any kind of work on this system you have to get the water under control and that means replacing the main valve. Is this your valve or the water suppliers? If its yours can you turn off your main valve to the property to allow the sprinkler system valve to be replaced and if its theirs or their valve (the main one) is no good either then get onto them to sort that part of the problem out. Once you have the water under control then you can repair away in your own time as you wish.