If you go to Dave Elliott's website
and click on "Concertina Maintenance Manual", then on "Manual Background and Content", scrolling down the page will reveal a photo of a couple of pages of the book.

The reed pan is the whole hexagonal wooden plate with reeds arranged radially around it, revealed once you have removed the end frame (left hand page shows a section, lower pic showing how the reed blocks may be removed). The reed pan is a tight fit in the bellows frame, but may be removed by inserting a finger in the central hole and carefully easing the pan out. Note there's only one right way to put it back! Usually the manufacturers stamped both the bellows frame and the reed pan with the concertina's number, in an angle of the frame/pan. Match them up when putting everything back together. Or add your own positioning mark before removal if you're unsure.

The individual reed blocks are the brass tapered blocks, each with two screws holding a steel reed. As somebody above suggested, don't mess with those screws unless you intend taking up a career in concertina tuning (or at least devoting more of your life to it than you would want!) With the reed-pan out, the tapered blocks may be removed for closer examination and dust/fluff/beard hair removal (Ouch! Sounds painful). For each note (in an English/Duet concertina) there are two reeds, one topside, the other below.

I'm not sure which bit you think is "permanently fixed". All the parts are designed to provide an airtight fit, so they may appear at first sight to be "fixed", ie glued or some other permanent fixing. But accessibility for repair/tuning demands that everything can be got at without recourse to breaking joints or doing anything unreversable.

I haven't got the book myself, but it looks like it might be worth investigating, even with thirty-odd years of messing about with these instruments behind me!

Ross