Dunnit!
Well RTFM, that seems to do the trick!
Doc1.doc = 1.97Mb
Save in .RTF format
Doc2.rtf = 7.08Mb
Save again in .DOC format
Doc3.doc = 1.16Mb
Which is a satisfying 41% reduction in size. So thanks RTFM! When I've worked out the parameters, I'll e-mail you a coconut!
Also tried:
Edit|Select All => Edit|Copy => File|New|Blank Document => Edit|Paste
Which gives about the same reduction in size, but changes the font and seems to embolden some of the text on a random basis.
And Roundtripping it through HTM produces an intermediate .HTM file of 4.23Mb (+ 123Kb "files" folder) and opening the .HTM file in MS Word and re-saving it as a .DOC file produces a file of 2.84Mb.
Thanks for all your help folks - as usual the PPrune community comes through!