Can I make the intersection of these 2 guides 0,0 or the ruler? 

0,0 will always be the corner of the page. 

You can set the page so that the origin is at the intersection of the two guides, and hide the page border/shadow.  I'd leave a hidden rectangle where the page will eventually be.

So, what is the workflow for creating objects relative to each other? or is it a case of having to use the X and Y boxes and lots of maths? 

For example, if I wanted a 3mm circle 10mm above that intersection? 

Maybe this:

Yep, that will work for my needs! 

Thank you! 👍