I know how you feel madmaz, I had a similar problem some years ago after a new build next door. We had about ten inches of water at the bottom of our garden for nearly two years. The builder re-piped the ditch, the council re-tarred the road and the water board admitted it was fresh drinking water but there was no leak, not sure how that works. Anyway, I came home one day and it was gone, never to return :-). Never did find the exact cause though.
I hope you get your problem sorted a lot quicker than I did.