Hi a blowtorch or if you don't fancy that Halfords do a freeze spray that works quite well, just clean the end of the bolt with a wire brush as that allows the penetrating oil to get into the thread, my preferred combination is Plusgas and the Turbo sockets that you tap on they have a sort of spiral grip system that bites into the nut and won't let go, sometimes they destroy the nut but thats ok if you have replacements.
Steve.