Wooops,
I guess I did not express myself quite clearly enough.
The far end of the wire terminates in a field , (and specifically near a pond)
visited by many cows.  In order to avoid any possible unpleasantness (wire
falling on the ground if broken by the wind, etc.), we decided to ground that
end.
It may well be that the whole thing operates as a loop of sorts, as there is a
non-zero resistance between the two grounds (the one at the Tx site, and the one
at the pond end)
73
Andre'

Dave wrote:

Surely the Voltage gradient is just the same but the other way round? High
current point at the earthed end and high Voltage point at the TX site as it
is about a quarter wave....
The "earthed at the far end" idea has been used with topband antennas for
years in order to get the current into the vertical drop.

73 Dave G3YXM.