Okay, I have looked and looked but not found. I need to use a very specific shade of Red for my logo, I have the RGB code but Inkscape seems to only allow me to use their swatches? Or is there a way to a custom colour? Thanks!

You need the fill and stroke dialogue. Double click in the bottom left of the Inkscape window, go Object-> Fill and stroke, or use the short-cut, control+shift+f to open it.

Then select your object, and use the RGBA: text box bottom right hand side of the dialogue to enter your RGB code.