Sorted it out - sometimes you can't see the wood for the trees.  Took
me a while to realise it but previous spreadsheet owner had set hugely
wide columns and massive text font but viewed at a zoom of 12% so it
looked like a fairly ordinary sized worksheet with an ordinary sized
graph.  When clicking print preview the graph mustn't be able to resize
itself in that scenario so the reason I was only seeing background was
simply because the print preview of the graph probably only represented
a small corner of the, say, top left or bottom right of the background.
The rest of the graph was "off screen" if you like.  Strange one, but
i thought i'd post the answer anyway, in case anyone else ever
encounters this same prob.