Ok, so after several more hours I am still getting WSPR spots from just G4KPX and G6AVK. Clearly the ERP with the present earth-electrode set-up in the new garden is in the low uW region only.
I'm going to try to string up a full wire loop for TX and see how this performs. I have a couple of options: (1) a thickish wire loop along the fence made from coax cable about 10m x 2m, or (2) a 1mm diameter PVC coated wire loop with a larger enclosed area. The former would be totally hidden and, because of the higher Q, may perform as well as the larger loop with thinner wire. The former is worth trying as it would be a very "stealth" antenna, totally invisible to neighbours ....and the XYL.