Thanks again Paddy, looks just like that, at least what I can see. I can see the sense in 'breaking the seal' and I reckon some GT85 will help. I'll let you know how I get on.
I'd like to save it if possible Paul but it's only a shim at the end of the day! I'm more interested in seeing it - it looks original but it certainly doesn't fit - its a bit of a mystery.
It was so tight it caused a just-visible bulge in the seat tube. I didn't think that could be possible but I googled it and sure enough Sheldon covers it.http://sheldonbrown.com/stuck-seatposts.html
After much thought I came up with a cunning plan. The saddle is held on by a barrel-like fitting.
Near where I work there's a backstreet with some round metal railings.
So I got a mate to help me clamp the top of the seatpost to the railings - him holding the bike in place so as to minimise stress on the tube from the weight of the bike. I took the wheels off but left the forks on which I regretted.
Once it was tight we set about pulling the post out. It took two of us holding the frame about 15 minutes. The screeching sound with every twist - a few milimetres at a time - was enough to bring people out from the nearby pub for a look. By then end we were exhausted and sweating and the last pull sent it into his leg and ripped his jeans.
I didn't imagine it could be so tight and have no idea how it got in there. We were careful to keep it in line with the tube as I was worried about the frame and I think the thick shim must have protected it.
No caustic soda or heat was used and it didn't take a week but I reckon it's a pretty good example of a properly tough seatpost removal! Pulling and twisting the frame with the post effectively fixed to a building must allow for quite a force to pull the post from the frame