I immediately phoned Thames Water whose first response was that it's my responsibility. They claim anything inside my boundary is my problem. I said you can't tell exactly where the leak is right now because it's underneath a brick wall. Eventually they agreed to send someone to take a look next week.
Here's a story worth noting...
We recently had a customer who was told by Thames Water that his water bill was unsually high due to a leak on his supply. They visited and did some tests and advised him where they considered the leak to be occurring: under his fully tiled downstairs cloakroom, under the stairs with mains entering under front door and under wooden floored hallway. They said they'd refund him the extra cost of water loss if he got the leak repaired.
He called us in and I wasn't convinced it was where they said, however, he asked me to replace the main and, as the unvented cylinder and boiler are in his garage, we agreed it would be better to run a new underground main outside, straight to the garage and leave the old main in situ and redundant, connecting the new to the old at the other end, just outside the front door.
This benefitted him as no disruption to interior decor, use of his home during the works and no more mains under the house for future problems along the run.
We did the work, as above and found the meter to still be running when stopcock in garage was turned off... as per my original suspicion: very common for supply to leak where water board(s) connect up their new "metered" stopcocks (during their upgrade initiative) to properties, just under the boundary.
Thames Water were called back in and dug up their meter on the public footpath (which only they can do) and indeed found it to be a poor connection at this location.
Longer story short... They undertook the repair (which they should have done in the first instance), reimbursed customer with the wasted water overspend AND, upon my suggestion, he was also able to claim back our invoice for running a new main, from them, as it was upon their guidance that he commissioned us to do the work.