Well, I finally beat the bugger. I had another crack at it on Friday evening, first of all trying to pull it out using the stem bolt with a washer over the end as suggested by
@presta . Of course the stem bolt threads weren't long enough to just use a washer so a big socket had to be employed which gave me a few turns until it locked solid.
A bit of head scratching sussed out that I'd run out of threads so another socket was added (the wedge must be about where the green elastic band is).
With the forks on the floor and a pipe over the allen key for leverage I managed to get the wedge a bit further down the steerer (at about a quarter turn at a time), but it was bloody hard work. All of this brute force, grunting & swearing eventually resulted in stripping the thread in the wedge though
. (I'm not sue which of the three actually stripped the treads). Plan B ~ drill the bugger out. Putting the fork steerer in the vice I got a decent sized hole (in a few bit size goes) in the main body of the wedge to start with (removing the wedge material right up to the steerer inside), hoping to get the hole large enough to reach the actual bolt hole but ended up about a mm short so I ran a bigger drill bit down the bolt hole, joining the two holes up.
The theory being the gap would flex and the wedge would come loose. Whether a cast wedge would flex I didn't know but a few belts with a hammer and socket bar resulted in the wedge eventually coming out ~ in three pieces,
result!
Now I can fit the a-head stem adaptor and use a proper, non-ski ramp stem for a better riding position.