My thoughts on the increased resistance are:

a) the extension is above new ground outside the limits of the earth mat

which covers most of my property.

b) the wood is rather dense and there are many smaller trees filling the

spaces, so it is very difficult to maintain the 2 metre separation
recommended by Laurie and others, especially when the wind blows!

Any thoughts?

Hello John,

The increase in resistance you encountered is probably not a real
groundloss but rather a 'thee loss'. If the antenna wire goes high up with
little distance to the greenery RF tends to take the trees as 'back-route'
rather than the air.
But despite the increased loss resistance your signal seems to be 6dB (1
S-point up), and after all that is what counts.
I am also suffering from greenery losses, loss resistance varies from 100
Ohm in winter to 140 Ohm is summer. I did a lot of test to work arround
this problem and found that the most efficient way to increase the
radiation resistance was to put the loading coil (or at least a part of it)
high up. The antenna current didn't increase (in fact there was a slight
decrease), but the signal went up about 4dB. So always do some on-the-air
testing to see if there is an improveemnt, don't just rely on antenna current.

73, Rik  ON7YD