Are there any materials exploring intermediate levels (of any sub-subject) which give plain English descriptions along side formulas/equations//diagrams/notations?
Because of the proliferation of notation across multiple physical specialties, I don't know if there is a concise reference. I don't know what level you are at, so I don't know where you feel comfortable.
I would start:
a) with a table of mathematical symbols
then move to:
b) Who is Fourier? as an introduction and desk reference on Fourier Analysis (and there are PHDs I know who keep this on their desk as well, especially once they discover who the lead advisor for the english edition was).
c) Quantum Mechanics Demystified by McMahon
d) Relativity Demysitified by McMahon
Do not be fooled by the cover art, granted these are not textbooks, but they will give you the basic training needed to step into more rigorous books.
I would then take time to understand what the Action is and what it means to vary it. One quick reference is:
e) Quantum Field Theory Demystifed by McMahon
f) Quantum Field Theory in a Nutshell by Zee
I would recommend starting with just the first few chapters of Zee before hitting the best kept dirty little secret paper on ArXiv that I have found to date:
g) A Simple Introduction to Particle Physics by Robinson, Bland, Cleaver and Dittman
Garanteed to turn you into a bed wetter.
From here you will have some interesting choices, but for recreation I would spend 4 weeks and watch and take copious notes (quite seriously) on all the lectures:
h) The Fourier Transform and its Applications by Brad Osgood at Stanford
You could probabaly actually just start with this, but it is important to make sure you take it seriously before doing so.
After all that, then I would start in on Supersymmetry, which has its own host of deep dark notation, as discussed in:
i) Supersymmetry Demystified by Labelle
and then you need to tie up everything you know and start in on string theory:
j) String Theory Demystified by McMahon
From here you should be able to read things directly in the arxiv, or dig into more rigorous textbooks.
This is series from Stanford university by Leonard Susskind:
which introduces and uses tensor notation. This is a fairly high level course with a good introduction (the first 2 or 3 videos!) to various mathematical descriptions.