I know nothing about the liability.
However, I'd get this done asap. leaks from water pipes cause all sorts of problems.
£1500 to dig a trench around a bunch of house, go under walls (presumably some of you have garden walls), shut off main supply, connect into each house's supply, reconnect mains and test, then fill in trench - that doesn't seem excessive to me. Not cheap, but not excessive. It won't be a 5 minute job.