Improve the section about pip version and using it.

Running-from-source.md

@@ -25,13 +25,26 @@ using the correct version.  You already need at least Python 3.8.x to use our
 
 ## Ensure you have working `pip`
 
-Ensure you have `pip` or `pip3` installed, check with `pip --version` or
-`pip3 --version`.
+Ensure you both have `pip` installed, and that it is for Python 3.x by running:
 
-On Debian you will need the `python3-pip` package installed.
+    pip --version
+
+The output will have "(python X.Y)" on the end, indicating the version of 
+Python it is for.  If this says "2.7", or indeed anything other than "3.X" 
+for some value of "X" then try:
+
+    pip3 --version
+
+On Debian and derivaties you will need the following command in order to 
+install this:
+
+    sudo apt install python3-pip
 
 Otherwise see [Installing pip](https://packaging.python.org/guides/installing-using-pip-and-virtual-environments/#installing-pip).
 
+Remember which of `pip` or `pip3` is the correct command for your system 
+when you get to [Use `pip` to install application requirements](#use-pip-to-install-application-requirements) below.
+
 ## Ensure you have dependencies installed
 
 ### tkinter
@@ -85,7 +98,7 @@ deprecated and we use `main` instead.
   * Note that on Debian (at least up to and including 10.x aka 'Buster') 
     you'll need `pip3 install -r requirements.txt` .
   * If you get 'command not found' or similar for the 'pip' command then
-     first try: `python -m pip --user install pip`.
+     try: `python -m pip --user install pip`.
 
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