Thanks for the various comments.
My proposed site is 50m from a single tree which is only about 5m tall. Other than the low stone wall (<1m) there are no buildings nor other trees within about 120m.
At this site I won't be able to fit guy ropes - the cattle in the field would trip over them and either pull the guys out of the ground or pull the mast down!
I have a 12ft long pole which is 42mm external diameter (1 2/3 inch). It is rather like a scaffolding pole but I think it is either an old handrail or piece of old iron heating pipe. It is quite heavy but also very rigid and will not sway in the wind.
I propose to secure it to the gatepost using two of these:
https://cpc.farnell.com/blake-uk/wb20-g/6-x6-standoff-wall-brkt-galvanised/dp/AP00962I may let the lower end of the pipe rest on a rock on the ground to take some of the weight.
To the top of this I’ll clamp a 6ft x 1inch pipe on top of which I can secure my Ecowitt WS90 (which is designed to fit on top of a 1inch mast).
With an overlap of 1 foot where the two poles are strapped together I’ll end up with a mast reaching 17ft (5m) which I think is enough. I’ll set correction to 10%.
With only 5ft of thinner pole being unsupported hopefully there will be little wind-induced sway. If there is significant sway I could lower the top pole and restrap the two poles with a much larger overlap and a shorter top section.
I’ll let you know…