When we bought our present bungalow 18 years ago, all the floors had to come out due to Red Ash infil (failed sulphur test) The hardcore was replaced with MOT (stone to me!) and the floors all re-concreted/screeded.
Obviously we were not living there at the time!
However, all the heating pipes were buried in the floor originally, and are the same now.
During recent work, I had occasion to isolate the cold water feed to the storage tank and CH header tank in the loft, it was then that I noticed that the header was gradually emptying (down a couple of inches in about 4 hours). It's not rocket science to figure out that I have a leak, but having checked all over the house, there are no damp patches in the floors etc. - Is there any way of finding the leak? the pipes are about 2-3" down in the screed, I think (they were lagged with something similar to hessian with a clear plastic overwrap if memory serves correctly). Or, am I going to have to re-plumb the whole lot "on the surface"?
Advice welcomed

Nothing on any of the overflows and no discolouration of the hot water (which I would have expected had there been a leak in the coil within the copper tank)
I can only think that the water is leaking from CH pipework undr the floor - possibly under the membrane? as there is no sign of damp anywhere. If it is under the membrane, it could be percolating away through the hardcor - 9 feet deep in places!

How wouild I go about testing the cylinder Scruff?

Many thanks!
- I'll do that and go from there.
As Arnie says.."I'll be back"

OK.
The only things fed by the storage tank are the feed to the cylinder and the bath tap. so, no problem.