Thanks for the info. Now updated to 8.5 and ... voila! It worked! When exporting as a PDF all upper case characters are now showing as they should with the accents in the proper places. I did nothing to the Quark file other than installing the update (I thought I'd just print it before I mess about with any of the options in the Transparency section).
Hmm, so what caused the original problem? Is my copy of Quark corrupted in some way? Can't see how as it's a genuine one and has never given me a moment's bother since I got it. Did the update somehow 'fix' a bug? Either way, I'm just happy the problem has been solved. Only took me 3 days of trying before I posted here!
To everyone who posted, many thanks. This problem was driving me mad and as my deadline is only 2 days away, I was beginning to think I'd never get this fixed.