I've been told that burning it will not destroy any seeds, and will send them into the air to spread far and wide.
And I've also been told that plants pulled at the flowering stage will progress to seeding, even after they've been pulled - and have seen this happen.
So I guess the best thing is pull in bud or in flower, but before seeding, and incinerate immediately?
The pull and burn - top - spray - sheep sequence makes perfect sense to me. I think I wouldn't follow the sheep with horses the first year, though; I'd wait to see what came up. Apparently it's very palatable at the rosette stage, when you can't see it very easily and so won't know that your ponies are eating it. And the seeds are very persistent, so perhaps it would be best to let the field grow after the sheep each year, then repeate the pull & burn - top - spray - sheep until it's been clear, or very nearly, for two consecutive years.
If you'd done pull & burn - top - spray - sheep, the grass which then grows, after pulling any ragwort that does come up again, would be safe enough for hay for sheep, would you think? So you wouldn't be wasting the crop completely while you eradicate the ragwort.