The questions you are asking are like page 1 of the manual. If you have questions like this you need to you need to go read the manual. VICIdial is easy to get around when you understand it, if you don't understand it you can seriously mess the settings up. Regardless your answers are below.
1) Does ViciBox contain absolutely everything we need to run the call centre system (minus hardware obviously)
Answer: Pretty much, unless you want a CRM or something to go with it.
2) Does ViciBox come with the web based interface system which i've seen on some ViciDial screenshots or is that installed separately?
Answer: The only way you interface with vicidial is web-based, so yes..
3) What is the URL to access the system after installation?
Answer: You will put in the IP address of the box you used unless you setup a domain. If you setup the system on a local network it might look like
http://192.168.1.XXX3) What is the URL link to PHPMyAdmin?
Answer: I don't think that PHPMyAdmin is included. Not sure never needed it.
4) Where are the PHP and HTML files etc located?
Answer: The web folder should be /srv/www/
Hope that helps and good luck!
ViciBox5.x86_64-5.0.3.preload from .iso upgraded to 13.1 | VERSION: 2.10-444c BUILD: 150129-0828 | 1.8.32.2-vici | Dual Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel twin quad core 3Ghz Xeon chips | 32gb of RAM