Ellie - 11:24 am on Dec 23, 2012 (gmt 0)
Thanks very much for all of the replies.
@Andy Langton - I think aiming to improve the authority of the ranking pages is a good option, although I'm hesitant to tweak the homepage too much in case we lose those rankings.
@Stru82 - Our navigation could well be overly optimised (we have exact match text, then synonyms phrases as title tags) so I'll try deoptimising to see if that helps.
I'll be sure to reply back in the New Year if we see any improvements. Thanks again for the suggestions.