Quote: ... I'm totally untechnical and wonder just what advantages a full frame has?
Two aspects 1 If you have a background in film SLRs, full frame gives you the same angle of view, Depth of Field as you used to get on film. Some people find this very important.
2 If your sensor has the same number of pixels but spread across a full frame sensor (as opposed to a smaller APS sized sensor), the pixel density is lower, and this means that your lens' performance will be better. (The higher resolution and lower contrast aspects of your lens simply won't be recorded.) However, towards the edges of the full frame sensor, the light rays will be more oblique and shading may (will) occur at larger apertures.
I have an EOS 5D and compared to my cropped DSLRs (including a 1D MkIII), it delivers good/excellent results with less effort. Visible on A4 prints, and easily visible on A3+ - but it does depend on my using prime lenses rather than zooms, even those which get good reviews.