It seems my settings weren't a million miles from where they should ideally be (and I'm sure further improvement is possible, and I will keep pursuing improvements), but in the space of an evening's work, the quality of parts that I am now producing has improved markedly.
Here is a photo of some of the various test pieces produced while working my way through the calibration process, tweaking the various SFACT settings as I went:
And to see how far I have come - here is the first RepRap TearDrop Shot Glass http://www.thingiverse.com/thing:11944 that I made about 4 weeks ago - one of my first "successful" prints:
Here's a rather better one I made a day or so later, when I had got my backlash under control:
It's still not perfect, but it is MUCH cleaner and "tighter"than my previous efforts - and I am seeing the quality improvements in my other prints as well