Ok, so as the final instalment of this test I have now got the wires from the 
earth rods coming to the transverter just resting on the wet grass (what was 
elevated by 1.5m is now zero). It seems to work best just feeding the 
transverter straight into the rods without matching suggesting low reactance 
and near 50 ohms. 

I'm still getting decent reports! So, it confirms the "loop in the ground" 
theory, or it is acting as some bizarre E field antenna. 

Will again leave it running overnight to get a decent number of spots but 
already I think we have an answer. 

What I can try next - is this ever going to end!? - is elevate the loop part 
"in the air" to  much greater height (about 9m to match the top of the Marconi 
I just took down) and see how much improvement I get. More in the air implies 
greater loop area so more signal. Just how much will indicate how much of the 
loop's effective area is in the soil/rock. 

WSPR spots please folks. 

73s
Roger G3XBM