Renaming files: Tag-Filename

I'm trying to rename files in the following way:
original filename:
01 Always and Everyone.mp4
I managed to get close to what I want with this:
%_parent_directory% %_directory% E%_filename%
A&E Series 1 E01 Always and Everyone.mp4
I would actually like:
A&E S1E01 Always and Everyone.mp4
I have tried:
%_parent_directory% S1E%_filename%
but I have to change this for each series. and I have approx 2000 videos to do this on.
I tried it with this:
%_parent_directory% %dummy% S%_directory%E%_filename%
but the %dummy% placeholder doesn't appear to work in Tag-Filename.
anyone have any suggestions?
Much Apretiated.

Could it be that the forum formatter ate up a couple of backslashes?
Because: right now

does not make a lot of sense in respect to use of variables.
(To display special characters, enclose the expression in accent-grave)
And: are we talking about metadata fields being used for the filename?
If so: why do you use the path components? They are already set and do not have to be written again.
Or do you only want to modify the filename without the help of data from the tag fields?

I'm talking about only modifying the file name because MP3Tag doesn't support AVI tags.

That is true.
But why do you write about mp4 files then? MP3tag supports mp4 files.

If you want to manipulate the filename there are also the functions Convert>Filename-Filename or Convert>Tag-Tag
Unless you tell us what the other fields (or pseudo fields) actually contain, it will turn into pure guess work.