Well that's a weekend I don't want to repeat.
Spent the last week overnight soaking in various solutions, coke, Ammonia etc. Riding the bike all week with the clamp not done up in the hope it might work loose. No chance.
Final attempt to get the tube to budge using a vice just rotated the saddle clamp in the seattube. Resorted to Stillsons on the post which just crushed it.
Spent Sat AM hacksawing away. Managed to get 4 cuts into the post lengthways but trying to 'fold' the seatpost just proved the tube was still stuck solid somewhere below where the top tube and seat stays joins the seat tube.
Had to resort to the Caustic Soda method.
Preparation is the key here. Completely stripped the frame. Sawed the post flush to the frame. I turned the frame upside down and bunged up the hole with big dollop of blue tac and then wrapped a plastic bag taped tightly.
I used a syringe to squirt the solution into the bottle cage mounts and then plugged the mounts up with more blu tac and plastic bag / tape.
I made a fearsomely strong solution, probably 50/50 volume approx. Water in a plastic jug poured in caustic soda then mixed with a wooden spoon. It gets warm. By now PVC gloves and full goggles are on.
Solution injected and sealed then I left for 1/2 hour until the bubbling stopped and cooled down. Some really nasty black stuff occasionally bubbled right 'up' the seattube and out of the hole in the bb shell. I just rinsed this stuff away when it happened. After 1/2 hour I inverted and poured the black stuff into a plastic bucket. Rinsed the tube and repeated.
The reaction is more fearsome with freshly made solution. Not sure why the solution would go off in 30 mins, or whether the reaction was better whilst the solution was warm.
I probably repeated this twice last night, then left it overnight with some older solution overnight reacting less violently, then twice this morning. The whole tube had disappeared. There's a little paint loss around the top of the seat tube. But considering the chemical attack...
Not something I'd want to do often but it's doable.