Hi terryplumb,
Thanks for the message.
Yeah, it's looking like it. Just put the water back on and re-measured, looks like the tank lost roughly 39 litres over 7h 20mins.
If my maths is right, that's 5 litres an hour or 120 litres a day or 3720 litres a month. You'd expect to see that amount of water appearing somewhere, but there's literally no sign of it (except on the meter).
Funnily enough, the wife's put her ear to the taps in the downstairs toilet and could hear a noise. She's got better hearing than me, but there's a definite sound of moving water - like a cistern filling (even though none are). Guess that might narrow it down a bit further?
Out of interest, have you seen (or dealt with) many leaks in concrete?
Off to check the insurance now... And look for a reliable leak finding person/firm.
Cheers,
pmb