Thanks for the reply.
Yes, they will be commercial water flow meters. At this stage, I was only going to utilise the one sensor/arduino to detect current water flow/24 hour average...given that we know a rough average water usage over 24 hours which is fairly constant, if I could detect anything exceeding this amount(with some overhead) over that period, there must be something wrong.
At the moment, (after receiving said large water bill) they are manually watching the mains meter tick over while they turn off different branches of piping to figure out which branch has the problem. Your suggestion however with having multiple meters on different branches would certainly make their job a lot easier!
I will get stuck into some code and report back.
Cheers,
Jason