I want to create a shortcut keyboard for the blur that is on the fill and stroke dialogue (Shift control F). Is there one already? I have not managed to find it.

Anyway, I have gone to:
edit > preferences
expanded the "interface" section and gone to keyboard shortcuts
in the search box, I have put in blur. Not sure what to do next as I do not understand the four things that come up. They are
blur
blur (no preferences)
Gaussian blur
Gaussian blur (no preferences)

Is one of these the same as the blur on thek fill and stroke dialogue (shift control F)?

The ones without the parentheses will open a dialog from the extensions menu. They are meant for bitmap images, which will be changed by it. The extensions have no effect whatsoever on vector data.

What might work (depending on what you want) is 'Filters: Blur (no preferences)'. You will need to use it as follows:

1. Open the dialog via usual way (Filters -> Blurs... -> Blur...).
2. Set a blur that you want to use for your objects (this cannot be changed when you do it via keyboard shortcut).
3. Set a keyboard shortcut for it in the keyboard map, e.g. Ctrl+Alt+Y (it's still free, I think, but you can search for the shortcut in the search bar, too).
4. Use the shortcut on the selection. You *can* use it multiple times, but this will likely slow Inkscape down more than setting another blur level via dialog, and the result will likely not be what you intended it to be (not a lot more blur per use).
(5. To remove a shortcut, hit Backspace when the dialog asks you to enter a new one)