I think most of us have had this problem.

Obviously WD40 is the first step and youve done that with no success.

Next step have you clamped the seat on and twisted to high heaven to try and budge it.

The you can always..now this may seem daft but try knocking the post in a bit further to try and shift the rust that is holding it, then soak again with WD40.

After that its the vice trick. use a heavy duty vice and clamp the seat post into it obviously ruining the post in the process, the grab the frame and twist like mad, this works on most occasions.

If still nothing and proceed with caution on this one, you can use gentle heating of the seat tube then twisting. this is a last resort and be careful as you can damage the frame.

Lastly send to an engineers and get it drilled out  

thanks for the info  raleighburner.com