the solution ive arrived at is this:
I built several wooden frames out of 2.3 metre pallets, using 2 inch x 3 inch timber as vertical corner posts, 18 inches high. I then cover them with orange debris netting, tied down into U staples with plastic cable ties, hammered round the frame bottom boards every 8 inches. The frames are therefore 2.2 metres x 1.1 metres x 500 mm high.
I then bed the frame down on the ground, and sprinkle slugs pellets round.
So far, after having several beds savaged by successive waves of rabbits, then pigeons, then butterflies, then slugs, this solution seems to work and has kept everything away from my spring cabbage, summer caulis and my bed of 96 turnips.
Ive now built a deluxe 3.3 x 1.2 x .6 metres prototype bed cover with an optional clear plastic cover (to make a cloche), additional vertical and cross bracing at every 1 metre, and plastic handles for movinging it. The plastic has net vents 12inch by 12 inch at each end to keep air movement and stop it getting fungussy. The only thing i have spent money on is net and nails.
I will also make a bubble wrap cover for it, as i have experimented with sewing bubble wrap (not easy!) on the sewing machine.
I tried a green plastic net with a 3/4 mesh, and my brassicas got severely eaten until i saw a butterfly fold its wings back and climb thru.