Can't find water leak

[gingerhead]

Hi
Can anyone suggest what the next step is?
Our boiler is has been losing pressure for a couple of months. It is now losing all its pressure overnight and has been for several weeks. We have had a new boiler put in and checked and tested several times.

No sign of any water damage anywhere but there must be a leak somewhere.
We've had track and trace out and they can't find anything using thermal imaging or through gas detection.
We are at a loss as to what to do next?

Our water company came to find a leak in our garden and the bloke had a shiny new bit of kit he wanted to try out. It was some sort of listening device gizmo and it worked like a charm.

Do you have pipes buried in the downstairs floor of the house, because that's where I'd look.

What is your water pressure like out of the kitchen tap - is that normal?

If it's losing pressure overnight you've got a major leak somewhere. If you're in the right area, hire ADI leak detention, I believe they cover London and the South East, they're incredible and use the technology that @DrJonesIpresume mentioned

Normal pressure out of the taps.
The pipes are under the floor - I believe we have suspended flooring downstairs.
I'll check out ADI - thanks. Although we are using a company recommended by our insurance.
We can't understand where the water is going as it's a significant amount every day.

Do you know where your pipes run? Have you turned off the water at your stop tap and checked the meter to see if it's still turning, a leak that significant I imagine it would be going round quite noticeably

Noticed any puddles or darker patches on your driveway/garden (if applicable) when it hasn't rained in a while? Or greener than usual shrubbery that might be getting a good drink from potential pipes running underneath.

I reckon it is a stuck-open or faulty valve.

Caveat - I know nothing whatever about plumbing.

What kind of boiler do you have? If a system boiler with an expansion vessel, top it up and keep an eye on the pressure. If it keeps rising, it may be an issue with that. It will rise until it's too high, then will release water, resulting in it having no pressure the following day.