Diagnosing my Bob Jackon seat tube misery - help!

RPM I've done that before..

and also done the inside of the tube, dowel with a slot cut in it to secure the emery paper wrapped round. then it's a bit like wanking for a bit, but it works even on steel.

but it's a new bike and BJ haven't bothered to ream it out properly

I won't argue with your wanking expertise, but I'd certainly be careful with the reaming/filing action bearing in mind how thin tubes are.