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My friend took 'The Builder's advice and got in a specialist company.
The system was first pressurised (again) and the central heating turned up fairly high. A thermal imager was used to ascertain the exact location/run of all underground pipes and the upper floors. The system was then drained and a gas mixture of 95% nitrogen + 5% helium pumped in. A sniffer probe was passed all along the known pipe runs. It was very sensitive to helium which would, if present, pass through concrete and laminates etc. for it to be sensed. The culprit finally turned out to be the drain valve in a small radiator in the downstairs toilet. It was leaking very slightly, so slightly, that the water evaporated before it could accumulate enough to be noticed, but enough for the pressure drop to affect the heating. The workers were on site from 09:00 to 19:00 and including replacing the whole radiator, it cost about £350. My friend was well pleased that no floors needed to be dug up.