Should I split it in 2 sections where I state my first 3-year marks
and then in a 4th year mark or just leave it as MPhys Physics and my
total average till now?
This is not a good idea for two reasons: one, the MPhys is an integrated degree; you don't get BSc + MSc, so you shouldn't try to display it this way on your CV. Put your final classification and mark if you have them, or a predicted classification if you don't (i.e. predicted 1st, 2:1, 2:2 etc).
Secondly, most (if not all) of the institutions you apply to will ask for an official grade transcript provided by your university. This means that there is really no need to go into detail about individual marks on your CV, as they will all be displayed on the transcript.
Also should I include all my experiments (which involved several
sessions of labs and a long report)?
Probably not. Listing every experiment you did as an undergrad will make your CV very cumbersome. Also, every physics undergrad I know did several sessions of labs and wrote long reports. Including this will not make your CV stand out. Pick one important experiment or project that you did well on and talk about that in your personal statement, if it is one of the factors that has motivated you to apply for a PhD.
Lastly, what would be the best option if I have no previous experience of a job?
If you don't have it, you can't write about it, but work experience in non-academic jobs is generally irrelevant for an academic CV anyway.
I'd advise you ask your personal tutor to read through your application materials, including your CV, before submitting. They will likely have some useful pointers, and it may help them too, if they are writing a reference for you.
