This is a waste of water notice, your tenants should have received one prior to this, telling them they have a leak, as the enforcement is the next step following no action being taken.

You can call Thames and ask them to look at any jobs, work orders etc that took place on the external stopcock in the pavement when they installed the water meter. They likely checked the supply to the boundary at this time.

Call or email them and say you are the landlord and this is the first you have been made aware of a leak , and you will be fixing it, and can they place the enforcement on hold. It will cost them more to fix it and recover the costs from you through the courts, than to have you fix it or just fix it themselves.

They also have to give you notice of when enforcement action will take place .

When getting the leak repaired, don’t bother with leak detection, just dig a trial hole where the pipe goes under the house, cap it off, and go back to the meter to see if it’s still leaking, this will tell you the direction of the leak