Hi Mike,
Thank you for the reply and offer of help, but I've figured it out after many hours of testing and comparing a file that opens with word online and one that doesn't.
Basically, I unzipped the .docx into a separate directory, deleted different .xml files in the "word" directory until I finally figured out that the string "xml:space="preserve" was the cause of all my problems, by deleting that string and recompressing back to a .docx worked.
I read up a little on what "xml:space="preserve" was for, here's a link for anyone interested :-
http://www.xmlplease.com/xml/xmlspace/The I changed the string to "xml:space="default" and recompressed, again this worked.
I then edited the docxvectorialwriter.pas file and changed every occurrence of xml:space="preserve" to xml:space="default (I think there were three), opened the fpvectorialpkg.lpk (in /Developer/lazarus/components/fpvectorial on my system), recompiled, ran a small test program and the resulting .docx opened in Word online.
I still have testing to do, but it does seem to work.
Sorry for the long winded reply, but I wanted to explain what was happening and the fix.