I think this is a common problem. Steel frame, aluminium seatpost. One stuck fast inside the other. Is it something to do with an electrolytical reaction? Sacrificial anode?
Whatever, this is how I tried to remove it.
Clamp the seat post in a large vice and try to free it by cranking it round. No joy.
Gently heat the frame with an oxy-propane torch. This time the seatpost came loose but sheared off half in the frame.
The heat burnt the paint off. But I’ve decided to strip her down to the bare metal and go for the polished look.
As for the sheared off seat tube, not quite sure how I’m going to remove that yet. Stay tuned.