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Andys - have you tried adjusting the Viewer settings, viz
If the settings don't help, then you could try one of the browser based epub readers (e.g. EPUBReader or Readium), if you find one that suits then you should be able integrate into calibre via its Open With feature.
FYI - there is a new version of the Viewer currently under development, consequently the current version is in 'life support mode'.
BR