I want to quit my PhD and it's only been 6 weeks Long story short, I realise that I do not like the field of study I have chosen and I Cannot see myself having the ability to successfully complete a PhD in it. I studied physics and went onto enrol on an applied mathematics PhD- whilst it doesn't sound that far removed from physics, I am a physicist at heart and I do not have any enthusiasm for the field I thought I'd love.
I guess the number 1 rule was to listen to my true feelings- I feel genuinely depressed at the thought of having to carry out the work.
Moreover, I feel that my supervision team have taken far too much on as my principle supervisor takes over a week to reply to things I send them, even after they request I do so. I feel that the best option for me is to resign and take some time to evaluate what I really want to study--I'm a physicist at heart and still want to pursue a PhD, just not in applied maths. My question is two-fold:

How do I approach this with my supervisor?
Is it okay to quit a PhD?
Yes, it's OK to quit if what you signed up for is really not what you want (professionally). Plenty of people who start PhD's don't finish.
That said, in your particular case perhaps consider finishing the year. Use the time to learn the kind of applied mathematics you could use when (if) you return to physics. It's too late for your department to use your slot for someone else. They might save a little money if they are supporting you - but if you are earning that support by teaching or grading then your leaving has  no financial value to them.
I don't think you should decide this rashly. You should be able to talk about it with your advisor. Perhaps try to set up an appointment (virtual if necessary) rather than discussing in email.