I just did as oddjob advises above and got my stem out. Soaked for probably 24hrs though, and scored the stem in the forks with a sharp knife to try let some pb down there (not sure if that helped but made me feel like I was doing something!)

I put an old wheel on and inserted old bars, then wrapped the wheel in a thick old sheet so I could pin it with my knees then twisted the bars. Took a lot of effort and a lot of wiggling but eventually got it free.