My money is in the under block and beam floor...so install a couple of ball o fix 22mm or 15 mm where the flow and return go in and out.Turn them to off. Put a pressure test on 2 bar for 24 hrs and see what happens if it holds its under the floor if it drops then let me know... two ballo fix dead cheap and take them out when you are sorted. Microphones huh window dressing for the desperate in my opinion !cheers for that. In a house with two teenagers and a gerbil, that's easier said than done . Leak guy is coming back tomorrow, I will suggest it to him (although he's already spent a day here with us all being out, and with sensitive microphones, and still not found it.