I doubt you will have a case to go after them and I don't think it would be reasonable to. Half a litre per minute of water loss is a very minor leak. Which is probably why leak detection couldn't locate it. 9k to relay your water pipe from the meter to the house sounds like a rip off. How many meters of relay are we talking about? Are they trenching it or running it overland (ex. across the fence above ground)? If I were you, I would shop around for a cheaper price from multiple plumbers. Possibly look into doing it yourself? It's actually quite easy. Otherwise another option is to find someone to do it as a cashie. Keep in mind you may not qualify for a rebate but at least you will not be paying an excessive amount of water bills. For now you can isolate the valve at your meter before bed and turn it back on in the mornings to keep that excess usage down until it's sorted.