Hi Gareth,
This is a pretty common issue.
Generally your homepage is always going to be your 'strongest' page for links etc so in most cases it makes sense to use it to target your highest level and most important keyword/phrases.
If you are ranking well with your homepage for this important phrase then you don't really want to be messing with it unless there is a good reason.
For the service page you could consider:
1. Leaving it as is since it serves its purpose for users and site navigation and conceivably supports the ranking of the homepage.
2. Consider slightly reworking to try target a relevant secondary phrase (ie bus rental instead of coach hire for example or private coach hire etc, etc) - assuming your homepage is not ranking as well for these type of searches and they are indeed relevant to your business.
The above assumes you ARE actually ranking well for "coach hire" with your homepage. If you are not ranking as well as you like then that might change your approach. Of course, if you did get the hire page to rank well then.... what phrase would you target with your homepage?
Hope it helps 