Slow water leak - nightmare situation - help!

Maintenance

My tenant announced a huge water bill rise and we checked it out (Victorian conversion block of 8 flats).   There is a leak between the meter (under a metal cover in the driveway) and the stop cock on the ground floor. This is the main driveway and tarmacked. The plumber sent by management tells me it's plastic piping buried very low and sussing where the leak is will be difficult. It may even run under a ground floor flat.  Looks like digging up the ground up and will cost!!  No other flat affected I know of).  Pipes don't travel together inside one larger pipe making it easy to pull faulty one out and replace. He thinks they travel seperately.

Question - I'm advised to contact a national leak detection company. No plumber can fix this - is this so?

I'm also advised I must bear the full cost - is this so?

Any advice would be so welcome as this all looks so awful at the moment.

Desperate of Southampton. xx