soak it in WD40, then drill a hole in the Seatpost. Insert a metal bar into said drilled hole and twist.
Failing that, clamp said bar and use the frame as leverage.
Firstly id like to say hello to all you bikers, iv just gotten back into biking after a 10 year visit to the Pub.
This is just a random post about my old Daves Chain Device. Were did they go? in my day they were cutting edge. =)