If you have the new type plastic valve that a meter goes on to, but no meter – can you test for a leak? Maybe Thames water can come and test if for me?

Test from where?
Do you think your supply pipe from the street to your stop tap is leaking?
Then Thames may be responsible, or, it could be you.
I had a leak in my garden on the supply pipe, they would fix it for a fee, or, they could fix it in a few weeks time when they can fit it it to their schedule. Anything beyond the street line is yours I was told. Unlike gas and electric which start at your meter.This was 10 years ago, so may have changed since.
Or you think you have a leak in your house?
If so, a decent plumber will be able to pressure test it for you.

Its leaking in the garden.

They’ll probably charge as anything over your threshold is normally your responsibility.

NB There are exceptions e.g. we had a disused water main under the house which sprung a leak and that was fixed for free……

It’s yours I’m afraid.

Give Thames a call, the guy that comes out might have a check meter to see if there’s anything going through your stop tap/boundary box. You may also get an enforcement notice if they prove a leak, also worth asking if they run a free repair scheme (Welsh Water do this)

Get a metal pole (golf club may work without the handle), put it onto the valve in the toby for your house. Try with valve open and closed (its a half turn valve likely) if there is a different noise open to that closed, you probably have a leak and its time to dig up the garden. You may be able to find the leak with your listening pole, carefully going up the route of the pipe. You could try divining rods, but this is fairly voodoo, so be careful of making a fool of yourself.

Try with valve open and closed (its a half turn valve likely) if there is a different noise open to that closed

This makes total sense, I shall have a go as soon as I get back.

Whereabouts in the country are you? I deal with leaks like this quite frequently for insurers.

If you are anywhere south of Oxford there is a company I use who for a flat fee (£650) who ‘ll send a chappy to hunt out the leak using gas tracer kits, listening sticks etc (and yes, some firms do use divining rods), and a plumber to dig down and fix the leak.

Let me know if you want the contact details.

Your water provider won’t be any help if the leak is downstream of the meter.