Thanks jfortner, your very last sentence "Update the 'Text Color' for (all) expressions" was the clue I needed to resolve my problem. Whilst this is a work around (for me) it has got the job done for now.
Selecting the Presentation option "Plot Values Inside Segments" does so for both (all) expressions and I only wanted to see the values for the first expression. Specifying the colour for the second expression (so that it matched the background colour) seemed to get ignored. It wasn't until I specified the colour of both (all) the expressions that it actually worked.
