There are no leaks, we have no tank in the loft and the central heating is a sealed unit. We can find any leaks anywhere and nothing is coming out of the overflow. Our neighbour has the same problem but says they don't know what it is either. Do we get the central heating checked and or who else whould we get to look at it? There seems to be no leak in the mains in the street either.

Turn ALL water off at the street and leave it OFF for over night. See if you can still hear water running. Make sure it is TURNED OFF at the CURB. Back to you. Milo

Another way to check would be to close all faucets and go out and check the meters little pointer. If it's moving you have a hidden leak.
Have you got a recirculating pump on your water heater? Let me know, Tom