Update: Problem fixed!
It came down to finding a bike shop with a reamer, which took a couple of phone calls. The mechanic cleared the seat tube down as far as the reamer would reach, and now accepts 6" of seatpost. (It seems difficult to describe this without it seeming rather suggestive, but perhaps thats just me....
)
Anyway, I'm now waiting on a few parts, and wondering what else I have in spares that will be acceptable.
Cheers,
Ed