Ah, understand. We believe the house earth rod is in 4" to 6" concrete, to it's top (path around house), the rest is in soil. Earth wire from the to consumer unit is prob 6mm but should've been 10mm. Plus wire is slightly broken in places. The new one I just put in for pond is 100% in soil with a 10mm Earth wire.I work in the power industry we have fun occasionally finding proper ground particularly when we are in an area with next to no soil. And it is "ledge " solid rock " bed rock " much the same issue your having but could be a number of causes