I recently found a 1.5 x 1.9 x 1.6m deep collapse in the field that was on the old German frontline at Bullecourt, straight out from the Digger Memorial but could not get anyone to pay any particular interest and as such the farmer filled it in. Damn, if only I was 20 years younger, I would have asked the farmer for permission to have a preliminary dig to see if there was a concrete entrance to be discovered.
I wouldn't bee too sure about farmers being as excited as you are, Peter.
Late 80's (when I was 20 years younger!) I found the remnants of a concrete entrance behind a farm near Menin Road, Ypres. Very excited, I started dreaming about exploring a forgotten tunnel full of artefacts, that had never been visited since 1918. To Boldly Go Where No Man Has Gone Before.
I couldn't imagine why the farmer never had checked the tunnel underneath the concrete. "And what am I going to find there?", he asked. "A flooded tunnel. And probably more tunnels. Pfff."
He basically didn't care. WW1 may be a hobby for us, for farmers it's mostly a nuisance.