This sounds daft but if you don't have a vice, try this - it definitely works.
After you have let the diesel or WD40 work its way down the seat tube, get an old stem and clamp it to the seat post. Lay the frame on the floor so the shaft of the stem is vertical. Get a piece of tubing or an old set of forks and slide it over the shaft of the stem, then start working the tube/forks back and forth (you need to stand on the frame to stop it moving so do it on a bit of carpet or something
). Once you get the post moving, you should be able to keep turning the post right the way round and, with a bit more effort, you should be able to turn it all the way out.
Using a stem doesn't usually cause a lot of damage to the post, so they can often be salvaged.
I don't have a vice or owt and I have used this method plenty of times...
