Last night I got it all apart and aside from a few broken glass bits I could find nothing at all wrong in the impeller/pump assembly. It was all suprsinglyly clean (probably due to the massive amounts of vinegar we've dumped into it lately!) Of course I can't rule out the serendipity factor here (as in perhaps I fixed something without knowing it!)

Thanks again for all help, perhaps it really is "rocket surgery"!

Bob