I've got approx a VW Golf bootful of Leylandii that's been seasoning in my back yard for a couple of years - I'll bring as much of that as I can fit in.
Raise the fire off the ground is the best way not to burn the field - alternatively cut and peel the turf back, drop in some paving slabs, burn on the slabs, clear the fire when done, remove slabs, re-lay turf - but that's a lot of work just for a night! I'll see if I can lay hands on a 20 litre veg oil drum we can cut in half and use as a firebox (or 2) restaurants usually bin the things. Kelley kettles or hobo stoves with a firebox/slab works too.
"Donít take life so serious, son, it ainít nohow permanent." Walt Kelly