I took the weedwipe course, but you have to do the basic storage and handling first, so it's 2 days.
Mainly common sense, but covers some things you don't think about.
If you can find a local contractor with a weedwipe, it may work out cheaper to use them, 15 acres is a lot of backpack work! (Guess how I know
). If the land is grazed down hard, the weedwiper only get the plants standing proud, so targeting herbicide use.
If not, by the sound of the number of your thistles the course is the only real way to go, but I dont envy you spot treating that lot. (If you do, i found running an electric fence wire dividing field into strips, saved confusion on what was treated).
Just seen Biw's post, I believe you can buy the stuff, but not allowed to use it without certicate, bit strange, but that's what we were told.