"Enquiries" or "Contact...

If my experience has taught me one thing, it is that you will never know for sure until you have tried, tested and measured the performance of each one. The comparable conversion rates will give you your answer.

Most times when you try to guess what will work best to convert your visitors, the "most obvious" choice will perform worse. I would love an explanation as to why, but I don't think there are many people out there who would even try to explain why this happens.

It depends on your target audience. If most of your visitors are from English speaking countries 'enquires' might work just as well as 'contact us'. For a more global website where visitors might have varying skills in English, 'contact us' will serve your purpose better than 'enquires'.

I would have thought contact us would have to be the winner,  

 "enquiries" just seems too clever and could risk people not finding the contact us...don't take the risk

The best thing to use it what your users are used to seeing.  So for me, 'Contact Us' gets my vote.  Enquiries isn't necessarily bad - it's just not as good.

You could have a link to your homepage of the site saying 'main' and people would be able to work out what it means - but 'Home' would be instantly recognisable.

Jakob's Law relating to usability and web visitor experience states "Users spend most of their time on other sites."  You don't want them to have to 'learn' how your site works, or what menu items mean.  I don't subscribe to everything Jakob Nielsen says - but I do believe Jakob's Law should always be considered when developing a site.