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Soak the bolt heads overnight with Kroil or another penetrating oil. Use a 6 point socket with a breaker bar or slip a pipe over the end of your ratchet to give you more leverage. They're not welded other than probably corrosion. If you live in an area that gets snow and ice and drive your Mini during the winter then chances are you'll have some corrosion. In the first picture the galvanized bracket looks corroded to me.