Database app, accessible from Mac, iPad and Web I feel very out of my depth with this query, but not being able to do it isn't really an option so I am going to have to learn how one way or another.

I have been tasked with building an application /  database for a Chauffeur company. I have done similar things before in Microsoft Access for other customers, hence getting this request, but this customer wants to be able to run the app on their Mac, and not install Windows. My only real experience of coding is HTML/CSS and some VBA when using Microsoft Access. For these Access DBs I have created separate front and back end files to allow multi user access and also remote access (the back end file being kept on the company server).

So onto my query (apologies for dragging it out)...

I need to be able to build something that the single user can open and run on his Mac, so he can view, add, change jobs and their details. He also needs to have the same access on his iPad, although purely viewing would suffice. 

As regards the web access, basically he wants to be able to go onto his "Booking System" application, go to "New Job" and send a link to his client, where they would click the link in a browser, fill in the details (Name, Contact Number, collection and drop off addresses, collection date and time etc...) and when they submit this form the details be updated on his booking system.

My issue is I do not really know where to start. What kind of database development environments are available on OS X (like MS Access is for Windows) which I could use as a starting point?
Short answer, grab FileMaker Pro. The iPad app integrates nicely with the desktop, and it runs on both Windows and Mac.

Additionally, since you are comfortable with HTML, CSS, and VBA, you might want to try NSB/AppStudio, which can use PhoneGap to generate a native iOS or Android app. We've created several apps to be deployed in the field for such things as product catalogs, quality checklists, etc. and it works pretty well.